Well, if practical, I would just remove that one board and replace it with a board with the tongue still attached. If for some reason this cannot be done, here are your answers:
1:you need a 1/4 inch slotting bit that doesn't have a guide bearing under the cutter, or over, depending on your perspective. The subfloor will prevent the use of this type of cutter.
2:you could use the pine you have at hand, I prefer 1/4 inch fir plywood, or a scrap of just about anything. Use a bit of glue if the piece is loose, not need if snugly fit, but nail it as though it came with the board.
3:1/2 inch expansion is probably less than your geographic region calls for, but in all prctical senses, if you maintain proper humidity levels in your home, that shouldn't be an issue.
4: can't help you there, I don't use either one so one of the American guys will have to help you with that.