It won't work.
If XT is adopted, XT wins, there's no other chain, or at least there's no other chain with value.
You may find someone that lived in a cave for the past year and scam them using the abandoned chain, but that's not something you'll want to brag about, I think...
It will work. That would be the case of reaching complete consensus with the economic majority.
AFAIK 75% (needed by XT) will cause a noticeable split, and thus this can be achieved. The guide is sound, and should work.
I understand that, I don't believe it can work because businesses will update their software in time, but if someone neglects to do so, it is possible and it can go bankrupt in a second...