Here's me getting it wrong again.
Indeed categories are fixed by Vat at MLM and landing config
So answers:
1. it does
2. it can
3. oh well, you can always use higher cat minima with lower cat speeds but not vice versa.
A320 could fly to Cat C minima when the IAS crossing the threshold would suggest that it should be flying Cat D??
Now that's the one that made me rant insanely and claiming that approach categories are not fixed. If you have to use speeds for higher cat, then you have to use minima for that category too. As for using lower category minima if your MLM is low enough to put your speeds in lower cat range, while I can't think of any reason why it shouldn't be allowed, it's very probable I don't know the whole story. Chances are that it's best to err on the safe side.
Apologies to everyone that might be mislead by my previous post.