... it appears that from slipping either the 'gear' moved forward so it was 1/8" from the from the extruder acrylic and not flush as described above or it was moved by the previous owner probably because it was slipping (LOL...a little tinkering require eh?).  I see from looking down the lowered bearing compression arm (tension arm as it has also been called) that with the 'gear' basically flush to the front of the extruder acrylic the knurled surface is lined up with the center of the bearing surface (probably why I noted that bearing slipping).

After some digging, this finally addressed my problem; that the knurled extruder gear was sticking out of the acrylic jigsaw by at least 1/16 inch. I had noticed this because my extruder was "jumping" occasionally, and a fine powder was building up on only one side of the filament drive gear. Evidently, the one side where the powder was building up, the far side of the gear, was the only part making contact with the filament.

Since the collar was sticking out, I easily accessed the set screw, but I was a bit confounded when I still couldn't move the collar. I unscrewed the four screws holding the stepper motor in place (leaving the jigsaw on the carriage). From jerseydevil's post, I assessed that there was little harm in getting out my vicegrips and trying to make it budge back down the shaft, since it seems like their collar had the opposite problem - too loose. Turns out it was just a snug fit, but through a lot of printing the collar had slipped down the shaft. I have replaced the setscrew and the stepper motor is re-attached to the carriage. Here's to hoping it prints well.

You got this fixed?

Yes I consider that issue resolved.  I still have much to tinker with to improve the quality of the prints but I get good positive traction now on the filament.

Here are pictures of my first print...

Please note that the one side of the cube that faced the bed looks like spagetti because large parts of it were unsupported by structural material (obviously not a great idea and more to do with the model than the printer or software).

A local hardware store to the rescue.  Lowes has M3 pan head holts but not cap screws and no nuts or washers of any type (makes perfect sense!).  Home Depot was a strike out.  I went down to the local hardware store and asked the counter and they had not just the nuts, but all sorts of washers which fixed this right up.

I left the 2 bolts that hold the extruder to the carraige loose and put in the 4 bolts that hold the stepper motor assembly to the extruder.  I left those 2 bolts loose as the acrylic holes for the stepper are very close and I wanted some wiggle room.  I put in 2 bolts in the 2 opposing corners but did not tighten them.  Then the other 2 bolts.  Then I cross pattern tightened.  When that was done I tighted up the 2 bolts that hold the assemlby to the carraige.  The 'gear' is stickng out about the height of a piece of paper from the extruder which has more to do with me putting it on the shaft when the extruder acrylic plates were not compressed by all the screws than anything else.  I was able to completely tighten the compression screw (tensioner) screw without the 'gear' slipping on the stepper shaft and no filament.

I can extrude with the compression screw all the way tightened with complete positive traction.  That is entirely unnecessary but just a test to see if this works as well as I expected it would.  In fact, it might be working almost too well.  Now I get way more than 1mm of output material when I extrude 1mm so now I must tweak settings.

I had raised the extruder temperature to 205 degrees previously because originally with a slipping extruder stepper I had to almost reduce the ABS to liquid to get anything out (yes I know that is not recommended but I have an IR heat gun and frequently turn off the hot end).  Now I have more than adequate force.  So I reduced the temperature to 200.  I also raised the bed temperature to 95.  I had the compression arm (tensioner) set to about 1/8" of the screw showing...which is very little pressure on the filament).  I just printed a 'companion cube' I sliced up from Thingiverse.  It's a bit messed up but I suspect the issue is more the way it was sliced than the printer.  All in all this worked out great.  I clearly got a nice even flow ouf the extruder.  Pictures will follow later.

You got this fixed?

A local hardware store to the rescue.  Lowes has M3 pan head holts but not cap screws and no nuts or washers of any type (makes perfect sense!).  Home Depot was a strike out.  I went down to the local hardware store and asked the counter and they had not just the nuts, but all sorts of washers which fixed this right up.

I left the 2 bolts that hold the extruder to the carraige loose and put in the 4 bolts that hold the stepper motor assembly to the extruder.  I left those 2 bolts loose as the acrylic holes for the stepper are very close and I wanted some wiggle room.  I put in 2 bolts in the 2 opposing corners but did not tighten them.  Then the other 2 bolts.  Then I cross pattern tightened.  When that was done I tighted up the 2 bolts that hold the assemlby to the carraige.  The 'gear' is stickng out about the height of a piece of paper from the extruder which has more to do with me putting it on the shaft when the extruder acrylic plates were not compressed by all the screws than anything else.  I was able to completely tighten the compression screw (tensioner) screw without the 'gear' slipping on the stepper shaft and no filament.

I can extrude with the compression screw all the way tightened with complete positive traction.  That is entirely unnecessary but just a test to see if this works as well as I expected it would.  In fact, it might be working almost too well.  Now I get way more than 1mm of output material when I extrude 1mm so now I must tweak settings.

I had raised the extruder temperature to 205 degrees previously because originally with a slipping extruder stepper I had to almost reduce the ABS to liquid to get anything out (yes I know that is not recommended but I have an IR heat gun and frequently turn off the hot end).  Now I have more than adequate force.  So I reduced the temperature to 200.  I also raised the bed temperature to 95.  I had the compression arm (tensioner) set to about 1/8" of the screw showing...which is very little pressure on the filament).  I just printed a 'companion cube' I sliced up from Thingiverse.  It's a bit messed up but I suspect the issue is more the way it was sliced than the printer.  All in all this worked out great.  I clearly got a nice even flow ouf the extruder.  Pictures will follow later.

You got this fixed?

I left the 2 bolts that hold the extruder to the carraige loose and put in the 4 bolts that hold the stepper motor assembly to the extruder.  I left those 2 bolts loose as the acrylic holes for the stepper are very close and I wanted some wiggle room.  I put in 2 bolts in the 2 opposing corners but did not tighten them.  Then the other 2 bolts.  Then I cross pattern tightened.  When that was done I tighted up the 2 bolts that hold the assemlby to the carraige.  The 'gear' is stickng out about the height of a piece of paper from the extruder which has more to do with me putting it on the shaft when the extruder acrylic plates were not compressed by all the screws than anything else.  I was able to completely tighten the compression screw (tensioner) screw without the 'gear' slipping on the stepper shaft and no filament.

I can extrude with the compression screw all the way tightened with complete positive traction.  That is entirely unnecessary but just a test to see if this works as well as I expected it would.  In fact, it might be working almost too well.  Now I get way more than 1mm of output material when I extrude 1mm so now I must tweak settings.

I had raised the extruder temperature to 205 degrees previously because originally with a slipping extruder stepper I had to almost reduce the ABS to liquid to get anything out (yes I know that is not recommended but I have an IR heat gun and frequently turn off the hot end).  Now I have more than adequate force.  So I reduced the temperature to 200.  I also raised the bed temperature to 95.  I had the compression arm (tensioner) set to about 1/8" of the screw showing...which is very little pressure on the filament).  I just printed a 'companion cube' I sliced up from Thingiverse.  It's a bit messed up but I suspect the issue is more the way it was sliced than the printer.  All in all this worked out great.  I clearly got a nice even flow ouf the extruder.  Pictures will follow later.

And now I laugh my butt off because I just noticed as I resinstalled the stepper motor that the 'missing' screw someone took out and put in the wrong hole long before me.  More over the nuts that hold the extruder on are no where to be found (they were never there).  Nothing like a tire with no lug nuts.  I guess the extruder stayed on merely from the cable tension before.  Off to go find me some metric hardware.

       I noticed my setscrew had some sort of threadlocker on it probably cyanoacrylate which bonded very well to the screw... so well in fact it make it hard to screw it further in the drive wheel - like yours - however once the threads were cleaned I had no problem threading it all the way into the inner bore. I will be adding a flat area to the stepper as long as it is apart as a lot of people I know in robotics do this because it makes for a more secure connection in high load situations, even though this isn't a high load area it does have a high heat/cool cycle and that makes things expand and contract which in time could loosen things a bit.

I removed the 3 (it looks like there should have been 4) metric 2.5mm screws that held the extruder stepper to the extruder assembly and having lifted some stray yellow Kapton tape I've slipped the stepper out the back of the extruder assembly and disconnected a few wires.  Having now reviewed this it appears that from slipping either the 'gear' moved forward so it was 1/8" from the from the extruder acrylic and not flush as described above or it was moved by the previous owner probably because it was slipping (LOL...a little tinkering require eh?).  I see from looking down the lowered bearing compression arm (tension arm as it has also been called) that with the 'gear' basically flush to the front of the extruder acrylic the knurled surface is lined up with the center of the bearing surface (probably why I noted that bearing slipping).  With the stepper now out of the extruder but still connected to the machine assembly (I disconnected the stepper from the PCB so it can't run as a generator) I am going to cover the machine with some wood or cardboard and use a rotary tool to clean up the shaft.  I used a 1.5mm hex key to work the set screw in and out of the 'gear' a few times till it further intrudes on the hollow in the 'gear' (I am reluctant to try to tap this hole and the set screw is plenty durable to work the threads a little deeper).  I can honestly say that while you can probably tweak the set screw with the ball ended hex key while the motor is installed in the extruder with an unflatted shaft I don't see how you can really expect to exert the torque required to dig the set screw into the shaft as this would require to be useful (you don't want it to score around the shaft just the spot where it was tighted because with no flat that's all you can do).

Like Gordym says it has no flat spot, this would be a great thing for the folks at SD to change or if you are adventurous with a dremel tool and small grinding wheel.....
The easiest way to get better access to the setscrew is to remove the thumbscrew/nut on the filament tensioner and that can hinge out of the way for good access to the right side of the extruder drive roller.

I thought the same thing but the acrylic outer layer around that 'gear' of my print head is closed on the side where that assembly pivots.  I could probably drill/melt a hole in there or grind away the back of that side of the extruder but before I do that I found some hex keys with the balls on the end that I will give a shot.

you would then eliminate all of the set collars. This might actually be cheaper or same price as the current setup