1a) It's up to you whether you actually leave.
2a) It's up to you which path you take.
3a) *? It's up to you for whom the bell tolls.
4a) * It's up to you the path you choose.
Why does it (maybe) seem okay to let "it" refer to [+wh] phrases, but not for [–wh] phrases, as in (4a)?
One also feels as though there might secretly be an omitted "as to" in there:
1b) It's up to you as to whether you actually leave.
2b) It's up to you as to which path you take.
3b) *? It's up to you as to for whom the bell tolls.
4b) It's up to you as to the path you choose.
all of which are now (maybe) fine. So, what is the function and form of "as to" here? Is it the same as in phrases like
5) I'm feeling a bit unsure as to whether this is even part of the same question, but I feel like the answers might be related.
(I'm not a full-fledged linguistics person, so feel free to simply point me in the direction of a resource that might help if this is too general a question!)