Like you, my spring popped out when I disassembled things to tighten that damn set screw because I didn't find the hole on the left side like you eventually did.
Unlike you, I wasn't able to get the spring back on (yet... I've seen a guide video now so I could probably do it).
I've been using a doubled-over rubber band over the lever and run down to a nub of plastic on the side of the assembly to put tension on that piece. In that regard, it's tension is adjustable. Looks bad though, and I dunno if the warmth inside there would ever cause the rubber band to fail when printing. If so it would just lose tension and it would likely stop extruding.
Even with my jury-rigging I still get the issue you're seeing... it will print for an hour or so then just stop extruding and print thin air. I have two half-printed Pikachu's that look quite nice. Like everyone else my wheel is grinding up the filament. When I get a clog I've been having to clear it with a piece of solid core copper wire jabbed down in the hole... I don't really like doing that though, I'm afraid it might cause some damage.
Would really like to get things cleared up to where I can get a full print without extruding stopping...