This is a good question - and I would like to have some hard factual answers to this also. @ncw is probably one of the few that would know for sure.
I think the only thing I can say I know for sure is that it is Drive2's upload quota that gets used, but this is probably fairly obvious.
Beyond this I suspect that it is Drive2 that pulls the files rather than Drive1 pushing them. I base this mostly on the fact that all errors I have seen from permission-related problem always seem to reference being unable to access the source - not being unable to access the destination. But this is just speculation on my part ... it would would not be to hard to experimentally verify this under controlled circumstances and looking at the detailed google API logs.