Very interesting -- I am more a Marvel fan and I don't quite see Cyclops as a fallen superhero -- not yet anyway but you made some really valid points in regards to the company he's keeping.
For me, its not so much fallen hero but characters that wobble on that line. And most fallen heroes end up dying and don't stick around for too long or like you said in Magneto's case, back and forth, back and forth.
Marvel couldn't outright make Cyclops a bad buy, and just like they did with Civil War, they made sure there was enough complexity (no matter how convoluted) to not have them go totally bad. Whenever a hero behaved badly, its mostly cleaned up with convenient brain-washing, Skrull impersonations, etc, etc. I think they really missed the boat with Sentry -- the Void was one of the best ideas for a villain and they just let that drop.