The only way that I can see that we end up with two chains is if the fork occurs without consensus. With BitcoinXT, the fork is programmed to occur with consensus, therefore that fork becomes BTC.
I don't think so. It only requires 75% of miners to agree. None of the other 25% of miners or any of the exchanges or merchants matter at all.
That's not consensus.
Even the recent soft fork required 95% before it was enforced:
when 950 out of the 1000 blocks preceding a block do have nVersion = 3, nVersion = 2 blocks become invalid, and all further blocks enforce the new rules
So why only 75% for this much riskier hard fork? Because they know it has no chance of reaching consensus.