1. It shouldn't do anything to the other lines since it's only upgrading on one of the lines.
2. Yes they will. They will give u the amount the system tells them to for the trade in offer.
Re #2 - do you know if they apply the trade-in offer immediately on the account, so it reduces the amount owing on the new phone? I'm thinking of trading in my iphone 7+ 256gb, so this might help with the current owing and keep me from having to cancel the order (although too late to cancel right now as it's in progress).
BTW, thanks Meowmix for all your responses. Much appreciated.