flywheel gives the engine it's rotating inertia. without it the clutch would have nothing to bite against when releasing the clutch.

if no flywheel then smooth pulling away would become a nightmare/impossible, the engine would most likely stall at idle (not enough inertia), the starter motor couldnt turn engine over (starter engages it's teeth on the outside edge of flywheel), and nothing for clutch plate to grip against.

skimming is definately nessercery on a flywheel if it's got too hot/clutch plate has worn too much. it warps, and no longer is flat causing clutch judder (felt when pulling away in 1st mostly).
too hot and it can crack; again not good.

 

Or since the ring gear is heat shrunk onto it, you can even make it loose and the starter just spins it!

I'm sure that would have revealed itself by now.

 

So can any one explain how a scroched fly wheel and worn clutch plate can lead to:

a) the car not being able to chang gear

b) the clutch not slipping (it was driving fine before it got stuck!)

Not doubting anything, just want to learn!

Cheers

Nick