Unless Grey's replaces Alex with a new actor (really, it'd be confusing 16 seasons in), they'll have to deal with Chambers's exit. Perhaps Grey's will close its Pac-North plot, returning Richard and Owen to Grey Sloan while Alex stays on board at the scrappy hospital. Alex could get mentions from Jo and Meredith if he remains at Pac-North. It'd be glaringly obvious that he didn't get a farewell, but this would be the least devastating way for him to go. We occasionally get references to Cristina, after all.
Alex could return if Chambers and the showrunners are up for it. Grey's has brought back past characters directly as well. We've seen Addison Montgomery return for guest-star episodes and Ben Warren dip in and out of the hospital as a firefighter. But it should be noted that these characters came straight back from Grey's spinoffs.