I can see what you are talking about - but it isn't what was originally being discussed. To recap, that was whether different actions are logically possible.

That seems like the same question to whether it is logically possible to have different worlds arising from the same initial conditions just before the decision was made - and according to the MWI, that is true: worlds branch.

The actions are the result of different calculations, sure - but the point is that before the decision was made, the world was in one state, and after it was made, it is divided into multiple worlds, with different decision outcomes in different worlds.

I classify that as meaning that multiple actions are possibilities, from a given starting state. The idea that only one path into the future is possible at any instant in time is incorrect. That is what quantum theory teaches - and it isn't critical which interpretation you pick.

For the other perspective see: http://en.wikipedia.org/wiki/Butterfly_effect

Small fluctuations are often rapidly magnified into macroscopic fluctuations.

Computers sometimes contain elements designed to accelerate this process - in the form of entropy generators - which are used to seed random number generators - e.g. see:

http://en.wikipedia.org/wiki/Hardware_random_number_generator#Physical_phenomena_with_quantum-random_properties

I don't think anyone is talking about impossible things becoming possible. The topic is whether considered paths in a decision can be legitimately considered to be possibilities - or whether they are actually impossible.