can't answer most of yr questions confidently, but as for 4. the answer is when u play something off Match on yr iPhone or iPod it DLs it and takes up space. if you haven't previously DLd it then if you aren't connected, u can see it in the library but can't play it. so u sort of need to be organised upfront and take down what u need for the flight.
as for vinyl rips, my experience is way less than a 50% matching rate i'm afraid. Match doesn't just use metadata (it's actually impossible to work out how and what it does from its behaviour). I think (but nothing seems to pan out consistently) there's a case for only trying to match a few albums at a time (u can open up another iTunes library folder, copy your albums to that then delete them from that folder as they match or upload), there seems to be an advantage to only trying once a day, and there seems to be an advantage to not trying to match ripped cds and vinyl rips together. but i stress that u never really work out whats going on and your first pass will probably only match a third of your library or something.
it's worth it once u have it done, but i would say don't match a whole huge library at once, its a process of a few months.
