Charles,
I checked the Zillions implementation for the answers, as the page is
somewhat unclear on some of them.
(1) Yes, you can capture an uncombinable piece.
(2) Yes, the captured piece is removed form the game.
(3) You must alway capture with the compound piece, as splitting may only
be done by moving to an empty square.
Split and capture would make an interesting variant. In that variant, my
answer to (3) would be:
Yes. If for you need to vacate the starting square (for example to give
discovered check).