If a person pushes a fat man into an on coming trolley and in doing so prevents
Good questions. The "fat man" and "switch" cases you described have been discussed ad nauseum in philosophy and more recently in psychology. These discussions have focused almost exclusively on the different intentions of the person deciding whether to act so as to kill one and save five. In "fat man" the person intends to harm and kill the one as a means to save the five (saving the five is also intended), whereas in "switch" the person intends to save the five knowing, but not intending, to kill the one as a side-effect.
I think this difference would in fact be significant in a court of law, where I doubt prosecutors would press charges in the case of "switch" (and I doubt juries or judges would convict--of what crime exactly?), but they likely would in "fat man" (e.g., assault and battery, perhaps manslaughter). I actually think the law would be getting things roughly right here. An important part of law is preventing risky and dangerous behavior. In "switch" there is little risk of harming more people than by doing nothing. In "fat man" it is far from clear that the person is justified (as much as he'd need to be) in believing he will save five rather than risking killing one as well as five dying. And in any case, we don't want to encourage people to risk killing other people when they think it might save more people (oops--but what about war?)
Alas, I've tried to test whether people's moral intuitions are being properly influenced by their epistemic intuitions (about whether the person is justified in believing their action will work), and the results did not come out the way I'd hoped (but they raised some other interesting questions). But your question raises another under-discussed confound in the cases. Typically we think that it is morally impermissible to disobey a law (at least a just law). So, if people are asked whether pushing is morally permissible in "fat man", there is one more reason to say 'yes'--that it is likely illegal--than in "switch".