Nuru,
Looks like you've been passed a bunch of information which should help - I would think!!
I'm guessing that you've applied liberal amounts of WD40 all over and around that collar. Have you tried to slide the collar foward at all? Or is it that the broken off plastic part is the only moving part aside from the locking ball bearings?
There has to be something else holding that silly thing on. I'd try to remove the U-joint and just have the yoke attached to the spindle. this will allow you some room to work so to speak. As suggested earlier, maybe by moving the the collar and yoke back and forth a bit will loosen up any burr or debris to allow the removal of the collar.
Good luck.
Terry