Ajax fan here. I think this deal is just a matter of time so congrats on buying potentially one of the best CB's in the world. I feel he is better than Jan and Toby were at Ajax although at the same time I don't think they are that comparable. Toby and especially Jan were at Ajax maybe a bit more technically gifted defenders, Sanchez is a very physical player but on the ball also pretty decent. Few years younger as well than Jan and Toby were when they left.
That probably won't happen. Although Ajax doesn't want to sell him, if he wants to leave (what seems to be the case) the club is probably open for negotiations. Nobody benefits from having a player around that doesn't want to be there (maybe Southampton disagrees on this). The only thing then left is for Tottenham to agree on a fee with Ajax.
He was left out because the manager decided he only wanted players around that were 100% focused on the match. Clearly there have been some talks between Ajax and Spurs, but apparently he had a chat with Sanchez first before leaving him out of the team.