No, many Germans speak fluent English.
Although many parts of the city are walkable, we would still recommend having a rental car.
Munich.
The wedding will be on September 17th in Garmisch. Depending on where you're flying from, the jetlag can be pretty bad. Allowing a few days in Europe to adjust is a good idea. We'll also have a few optional groups activities on the days leading up to the wedding. If you want to be a part of those, then plan to arrive in Garmisch 3 days before the wedding.
Yes! Oktoberfest takes place in Munich. It is a short train ride and a walk away.