In and around Munich. Should I focus on Andechs or Weihenstephan if I only have time for one or the other? Additionally, are there any bottle shops around Munich, Freising, etc. that might have Westvleteren bottles for sale?
Well if you're just interested in the beers, Andechs has a bar right in the middle of the tourist center, so you don't need to go out of your way for those. Also Weihenstephan appears to be easier to get to via a simple tram ride. I have never been to either, though, so if you're looking more for ambiance and biergarten character maybe someone else can chime in. Re: Westvleteren, the short answer is "no". I don't even think any of the Brick Packs that were sold throughout Europe (and later the US & Canada) were made available in Germany.
Highly unlikely to get Westy in/around Munich. You can get Andechs downtown, at Andechs am Dom, saving a trip to Herrsching. You could then head out to Freising and Weihenstephan, taking in a pub called Schneiders also, where you can try a number of local Bayern Biers.... Prosit!
I found Weihenstephan a huge diasappointment. It's basically stuck in a suburban light industrial estate. Not very romantic.