made a tutorial for masking:
https://www.youtube.com/watch?v=uEEdGQLVnxk
but the green screen thing is still not really solved, ended up with rendering out the whole thing 2x, first time with the green screen applied, then doing the masking stuff… 
a few hours later, masking the simple way (thanks to @Shrinks99):
https://www.youtube.com/watch?v=H947uhij76o