I'd suggest attaching the INF ticket to one of the coach tickets - you will have to pay 10% of the full economy fare, which will be cheaper than 10% of the full business fare.

As far as switching seats, you will have to decide who sits in the business seat before takeoff (doesn't necessarily have to be the person who is holding the ticket for that seat). Changing seats mid-flight is typically not allowed but you may get the crew who will let you do it once. Forget about the frequent swaps. I would also discourage frequent visits from the back cabin out of courtesy for your fellow passengers.