Planning next years trip and was wondering;

1) can I use a 241 with 3 pax on a booking?

2) can I book one way for the first leg (west/east usa so little availability) and add the second one on for just one fee? At £35 per person? Or would it be the offline charge for the first and second leg as well as the charge to alter which I think is £35pp?

3) is there any way to get bookings linked/ seats if I book the one silver as a UUA and the rest on a 241, as I would like to avoid the extra costs for seat selection as they will drop from silver before this flight. (I realise these can be booked when silver but it's common for a plane change on the west coast routes.

Thanks