What is a REASONABLE number of days to see these Southern bavaria attractions?

Judy, we are in Germany at the moment and visited most of the places on your list in the last few weeks. We have a car so no problem getting to places and saves lots of time.
We stayed at Oberammergau for a few days , its only a small town so a you can generally look around in the afternoon /night, if you are visiting say Linderhof and the Ettal Monastery ( I think you can catch a bus to there from Oberammergau)
we spent the morning at Linderhof , then called into the Monastery ( only about 1/2 hour there)
we spent the best part of the morning at Zugspitze then drove via Mittenwald ( another small town really) on our way towards Hall in Tirol.

we did Eagles Nest, Konigsee lake cruise and walk, then the Rossfeld road all in one very big day ( due to beautiful weather, and rain the next day) while based in Berchtesgaden. ( perhaps think about it as a base? for this par to Austria)

a lot of the towns are only very small and don't really take all that long to look around, we tend to do them in the afternoon, before dinner etc as its not dark till late. the shops stay open to reasonable hours.
you will need to allow more time as without a car I imagine it will take a lot longer just getting to the place and then on to the next.

When planning,we found the difficult bit is trying to narrow down the 'nice' places to visit, rather than the ones that seem to just pop up in the brochures or the net and being happy to base yourself in one place for a few days so you can change you plans to suit the weather, if needed. Rather then moving around all the time.

hope this helps

I would base in Oberammergau for part of the trip. You can leave Munich Hbf at 9:32 using a Bayern-Ticket, switch to a bus at Oberau, and get to Oberammergau at 11:19. If you stay here, it's a short walk to your accommodations; check in, leave your luggage, and get back to the station for the 12:50 bus to Wieskirche. That will give you an hour to see the church before catching the 15:00 bus back to Oberammergau (15:34). Spend the rest of the afternoon and evening exploring Oberammergau and having dinner. Note: there is really nothing extraordinary about Wieskirche. It's just like a lot of other overly gaudy Rococo churches in Bavaria. When I was there in 2007, I walked in, looked around for a few minutes, and retired to the Gaststätte across the street for lunch.

The next day, take the morning bus to Linderhof. I found the grounds fascinating and spent most of the day there. Some of the buses return to Oberammergau via Ettal. Guided tours of Ettal are only for groups, so you probably won't be able to see anything. It's a cute town for a quick visit.

The next morning, check out and take the bus at 9:30 getting to Garmisch-Partenkirchen Bahnhof at 10:16. There are lockers on the platform at the Garmisch-Partenkirchen Bahnhof. Stash you luggage there and walk over to the Zugspitzbahn Bahnhof and take the next train. Get off at Eibsee and take the cable car to the top. When you come down, take the train 21 minutes to Mittenwald.

It's worth spending the night in Mittenwald. It's not a big town, but enjoy the ambiance There is a nice pedestrian shopping street with plenty of restaurants.

The next day, take the train from Mittenwald to Berchtesgaden (change at Garmisch-P, Munich, and Freilassing). That will take almost 5 hours. Take an extra hour and have lunch at the food court in the Munich Hbf. I would spend*** at least*** two nights in Berchtesgaden, there is a lot to see there.

So that's Oberammergau, 2 night, Mittenwald, 1 night, and Berchtesgaden, 2 nights.

Now, the monkey wrench in the works- weather. Two of the destinations on your wishlist are highly dependent on the fickle mercy of the Alpine weather gods, Zugspitze and Kehlsteinhaus. I would visit these at the first opportunity the weather presents.

Linder how had extensive grounds, so this would be better seen on a dry day as well.