In these cases, the hypotheses of von Neumann-Morgenstern utility theorem don't apply: the set of choices is unknown and so is the ordering, even on the elements we know are members of the set.

It seems to me that you give up on VNM too early :-)

1) If you don't know about option A, it shouldn't affect your choice between known options B and C.

2) If you don't know how to order options A and B, how can you justify choosing A over B (as you do)?

Not trying to argue for FAI or against environmentalism here, just straightening out the technical issue.