Also, don't the September rules specify that of the two extra guys, one has to be a position player and the other a pitcher? If so, I don't think they'd be able to replace Alvarez with a pitcher.
The Braves may hold Sale for Game 2 and if they win the first game they'll certainly have someone else start the second game. And I assume the same would be true of Schwellenbach if Sale starts the first game.
I don't know if it's been noted here before, but like the Mets, the Braves also have the tie-breaker advantage over Arizona. So if we end in a three-way tie, the Diamondbacks will find themselves on the outside.