Repetier Questions

It would be best to completely flash the firmware via the board jumpers, and uploading custom firm using Arduino IDE (the da vinci's board is based on Arduino Due)

There are some guides out on the internet, but most of them are not clear.  I might personally make a video myself with STEP by STEP instructions, but I need to gather my resources first.
Here are some answers to your questions...

1) Reverting back to stock firm:  Right now "The Pit" has a video on how to revert back to stock for the 1.0, assuming that you have a different printer, you can find other firmware versions online, usually at Voltivo forums.  Here is the video link: https://www.youtube.com/watch?v=sU5G0EX_Q08

2) As I said before it is best to flash the board via the jumpers, and upload it using Arduino IDE

3) It will make the printer BETTER, the only thing it loses is the cleaning tower feature, but you can always add custom G-code to get it back (I only use the wire scraper).  Using repetier firmware and host will give you FULL control over what settings you can use, anything from temperature to speed to advanced infill settings (even more).  The LED lights will still work, you can turn them on or off, I usually keep them on. 

4) It will not randomly turn off or mess up, not that I know of.  The only time it crashes or messes up, it's on my part.  Sometimes I accidentally close the software and the print cancels, or the computer sleeps and the Repetier stops the print.  If you leave it alone and nothing interferes with it, it will run smoothly.

5) Again I will soon upload a video, a much clearer guide.

Hope this helps, if you have an more questions just ask

The moment before the start of a print when the print head moves back and forth on the little scraper in the corner to remove excess filament, it's never useful anyways (I use the copper brush or tweezers).

As I said you can add some custom g-code if you choose but it's just a waste of time.  I am using Luc's Repetier RC1 firmware for da vinci with 2.0 setting.