As for a sign and trade, it should be noted that the Kings CANNOT sign and trade Thomas if he actually signs an offer sheet. Per Larry Coon's CBA FAQ on qualifying for a sign and trade the following must be true:

(UPDATE 11:00 AM: Ok so I guess there's a little more here than meets the eye. Apparently a player can sign, but as long as the team he signs it with doesn't give the offer sheet to the former team, in this case the Kings, a S&T scenario can still be worked on since it's not technically official until it's been handed over. This is apparently what the Mavericks and Rockets are working on with Parsons right now and he has already signed. )

Speaking of teams Isaiah could want to go to, here are the teams that can offer Isaiah more than the Mid-Level Exception (i.e. more than the Kings would probably want to match) after some signings already this offseason: