Joey
Toto Wolff recently said all drivers except Raikkonen & Kyvat have contacted him regarding the seat ..
Alonso can't go, because Mclaren have him contracted for next season, Wolff himself just recently mentioned that while he didn't rule out a legal battle with another team for a driver, he wouldn't want to pursue it if it can be helped unless he had help from the other team to avoid a legal battle. for mclaren, that is not likely to happen as mclaren know what Alonso is worth.
Wehrlein was actually close to a deal with Sauber to race there for next season but those talks have stopped due to the unique situation with the vacant Mercedes seat. It's possible that this Mercedes seat could be taken by another driver and he could still race in Sauber. However i doubt Koltenborn is willing to wait for very long to commit her driver line up.
Vettel has contract at Ferrari and made it clear he isn't going anywhere by letting the media know about the contract.
Horner mentioned that both his drivers are locked into multi year deals and he knows he is still on a gold mine, no way he would willingly let a rival team take one of his two most prized drivers. You can be sure Toro Rosso have the same arrangement through Marko.
Just because half the grid has contacted Mercedes for the seat, doesn't mean half the grid are willing to break their contracts to get to it and then put themselves in a huge legal battle in court.