The XT chain will quickly become polluted with outputs which are invalid on the BTC chain. "Taint" from coinbases generated only on the XT chain will fan out.
Yes; just as any transaction that uses utxos tainted by coinbases of blocks in the small-block branch will execute only on the small-block chain.
But
only sophisticated users will know that, and only they would be able to exploit that feature to work with each coin independently. (How would
you get your own pre-fork coins tainted that way?)
Everyone will know it either before any fork or very shortly after. I've described the formula several times to spur people's interest in learning the basics of how spends are created. Hopefully a few people will have taken and interest and took the 5 minutes it takes to understand this, and many more will be aware that it is a TODO before they get ripped off by on-line service providers or dodgy SPV client apps.
I chronically overestimate people's abilities, but I am quite sure that this stuff is simply not rocket science. Someone will have written some code to auto-split coins well before it's needed (if they've not already) and if they don't, I'll start coding again for the first time in a number of years. IIRC, some of the same techniques were employed to either exploit or work around exploits associated with the mutability issue.