Entering Ukraine without a return ticket

Dear friends

I have a question which I think nobody can answer for sure, but I would appreciate any thoughts on it.

I am a Brazilian, my fiancée is Ukrainian. I have been to Ukraine twice, both times on the 90-days within a Protected content period which is the usual deal. Next year, early in February, I intend to go to Ukraine again, so we can get married.

My question is: do you think it very likely that I may be denied entry if I arrive with a one-way ticket only, or with a return ticket that exceeds my 90 days? I know this question sounds a bit naive, but I have no idea how tight Ukrainian entry control usually is. My idea is to apply for a visa extension once I'm in Ukraine, but perhaps the best course would be to ask for one beforehand, from Brazil...

Well, I know this is all a bit vague, but I'd appreciate tremendously any input. I'm at a bit of a loss here.