No, you can sign a player to an extension after a trade is made. That's how it's always done. Sometimes a selling team will give you permission to talk to a player beforehand but that doesn't change their trade value.
Anywhere in the West is long travel but you are nuts if you think Treliving would ever trade him within the division. History? What does that matter? There is no state tax in Seattle, the cost of living is significantly lower than places like LA, Chicago, New York, or anywhere in Canada. I don't think they are underwhelming at all. I think Beniers significantly underachieved this year and with 23M cap space I think they could absolutely be a playoff team next year with Marner