id imagine that "any attempt to hold a referendum would be illegal and they would not acknowledge it" is because you cant be sure it would be fair while Russia is occupying the place and potentially fixing the result against you
the answer as I see it is for Russia to back off (even just back into their Crimean Bases) on the agreement that a fair open vote is held about the Crimea - no one can cry foul then
The trouble with that is Russia doesn't like or do fair open votes.