Time: sure.
Reputation: Actually, I think going above and beyond for a stranger in need is pretty strong signalling of my Altruism, especially among the communities I walk through.
Legal risk: This is what I want to hear about. What legal risks?
I'm assuming you're asking about a sham marriage that you're going to end later.
Maybe, but I don't see any particular reason to get a divorce except 1) the refugee in question wants to marry someone else or 2) so I can go marry another refugee.
It looks like marrying specifically for US residency purposes is illegal. This report gives the impression that only a tiny fraction of people actually get prosecuted. You'll have to convincingly lie to a consul and likely undergo some investigation (see e.g. here).
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.