I don't know what happened with Alex they heavily promoted Flight 462 saying that a character from it was going to be a main on fear than Alex appeared in 2 episodes and was never seen or mentioned again
Season 3 never brought her back
Than again seasons 1-3 of fear forgot about so many character
Tobias
Alex
Seth
Diana
Jack
Vida
And so many others
As for Proctor John, Walker and Crazy Dog I think they would've come back but because the show changed direction that's probably why they haven't showed up.