A:AnswerLog on to your service provider (eg, Verizon) web page. They should be able to give you a trade in value for your phone. Once you get beyond 2-3 years the value drops off to almost nothing so don't be shocked if that's the value you end up with.
A:AnswerYes you can use this deal for your S9. I had a S7 and it does get sent in after you pick up new phone. They will check your phone for damage but if not damaged, you should be able to get around $200 once you sent in phone.
A:AnswerYes. I know someone who did earlier today & it was no big deal for them to keep their old phone & choose not trade it in. They were told their account wouldn't be charged until the phone ships.