1) if you book three seats, off which two are using avios and the third is "free" using your 241, you will end up with two separate bookings - one for a single seat, the other for 2 seats using the 241.

2) I think (but have not yet tried it) that adding a return leg because they dates were out of range (more than 355) is not subject to the change fee.

3) not that I've ever been successful with. Best is to make the silver holder one of the pairs on the 241 (unless you want the miles from the paid ticket)