Tax filing, civil unions, joint or single? help!

I am trying to figure out the rules regarding filing US taxes when living in Germany permanently via partner sponsorship but there are many things I am not sure of.

1. civil unions (same sex) are not recognised in the US--would I file as married or single? Single right? She has never been a US citizen, so I am assuming single filing.

2. if i'm unemployed, do i still file? if i receive income as a student? will i have to file even once i renounce US citizenship?

i want to know these things before i move, but information on this specific issue is hard to find. and frankly having to pay taxes outright instead of having them automatically deducted from my checks sounds scary.