And all user coins can't be returned because there's no way to do a rollback and return all valid coins.

You have better technical knowledge that I do but couldn't they just "refund" these coins using the premine if they really wanted to ?

Funding isn't the issue; there's no way to validate what is/isn't in people's z_addys (that's the entire point of them).

Right they could refund t but not z, makes sense.