(Just to be sure, I expect this is exactly the point you've changed your mind about, so there is no need for me to argue.)

My default is to assume that B utility cannot be produced in a different world UNLESS it is of utility in B's world to produce the utility in another world.

Does not compute. Utility can't be "in given world" or "useful" or "useful from a given world". Utility is a measure of stuff, not stuff itself. Measure has no location.

Your assumption seems to be that B utility will always have value in a different world.

Not if we interpret "utility" as meaning "valuable stuff". It's not generally correct that the same stuff is equally valuable in all possible worlds. If in worlds of both agents A and B we can produce stuff X and Y, it might well be that producing X is world A has more B-utility than producing Y in world A, but producing X in world B has less B-utility than producing Y in world B. At the same time, given amount of B-utility is equally valuable, no matter where the stuff measured so got produced.