Bitcoin XT is the only viable implementation of BIP 101
It is not. Any miner can make a copy of XT and remove the patches that he objects to. Or make a copy of Core and add only those XT patches that implement vote-triggered 8MB limit.
If you are not a miner, you (or any programmer you trust) can make a copy of Core, simply raise the size limit to 8 MB, and start running that version any time before the fork actually happens.
I'm not sure the masses of adopters are gonna do this / or even know-care about the possibility. Vanilla XT is the intended norm.