If he leaves for a club like Liverpool then he'll be in Bayern in 3-4 years anyway replacing Neuer.
Doubtful. They're just in the process of extending Neuer's contract until 2021 (not sure if it isn't already done). He'll be 35 by that time. Oliver Kahn retired at 39. MAtS will be 29 in 2021. With the amount of young German goalkeeper talent lining up I doubt they'll sign a 29 year old, even if Neuer really retires then.
If MAtS is benched for the CL games, I doubt he'll have a chance of getting into the Euro 2016 squad. Playing only the Copa del Rey won't be enough. There's too much competition. There are Trapp, Leno, Fährmann, who are all playing week in, week out and are in European competitions. Then there's Zieler who has the advantage of being already in. Heck, even Horn might get in ahead of him.