Alot of questions there Adam.
Firstly, to remove a piston from the top, you need access to the bottom end, hence why engine should be removed.
If there are bits everywhere, more than likely the above will need to be done.
If the guy's bottom end siezed, don't touch it!!
Hopefully that answers most of your queries
EDIT: If you remove the piston in question and it looks OK once you've cleaned all the shit out, there maybe a case for using it again......if the piston is damaged however, changing just one piston may also be a possibility. Without sighting anything though, we're pissing in the wind unfortunately.