There is enough work already done in game theory about the dangers of nuclear escalation. Current state of affairs pretty much ensures no rational agent will want to use them. On the other hand, when you model the interactions of significantly more intellectually capable rational agents with us, the "calculus" is not very favorable, to put it mildly.

Obviously. No company would practically have been able to do it, due to the investment required, but there would be no laws stopping it.