No reason to turn down the extra sales, there's no guarantee everyone who bought it for XB1 would have bought it if it were only on PS4
depends on if it was difficult to port it at all. One would assume it wouldn't cost them that much, once the game was already made, to release on btoh consoles. But supposedly some developers have claimed from time to time that the process can be more expensive than expected
I doubt the PS4 and X-ONe architecture is that different though so it would seem like an obvious good choice given 1 million extra sales or whatever.
That said, Square Enix bitches about what appears to be great sales in the millions ALL OF THE TIME so its quite hard to tell what is or isn't worth it to them. Based on their early predictions (which I think was like over 10 million for the game to make money apparently) they'll probably be disappointed regardless of what happens