In 2016 I travelled extensively in Switzerland and in 2017 in Switzerland & Germany. I did not user Eurail passes but found it much cheaper to buy a Swiss Travel Pass and a German Rail Pass. They are effectively a single country pass and work the same as a Eurail pass. They also give better discounts on mountain railways, cable cars & boats than the Eurail pass.
I have NOT travelled in Austria but it is in planning. I have found the Austrian Tourist Office to be very helpful [the one in Sydney anyway]. There is an Austrian rail pass and they have very good information on OS services.
Some other comments:
*Vienna to Milan is about the LAST route in Europe I would travel overnight. You will go through some of the best Alpine scenery in the dark, what a waste! If I was doing that run I would use the Nightjet from Brussels to Cologne/Frankfurt, if there is one. You'll make up for that scenery in the boat trip from Koblenz.
*Walk up fares are VERY expensive in Switzerland and expensive in Germany. The Swiss have a Half Fare card, which does exactly what it says,, it cost about $80-90 USD, Germany has something similar but I have not used it. It is also valid on even more transport options than the others. In 16 I made the cost up in two trips . Suggest you look into these for the countries you are visiting.
*I found DB timekeeping very good [generally not as good as Switzerland, but even there I had a 20 minute delayed connection in Spiez at 2C with a substantial wind chill, so even Swiss railways can have problems!] Also a missed connection in Germany is really not a problem, there will be another train along within 30 minutes on local/secondary lines and 10-15 minute on main lines.
*About that "just jump on any train" thing. It's true IF there is room. In Germany no domestic train requires a reservation but most main line trains [ICE, Intercity, etc} CAN be reserved. If you get on a train, even ICE and there is no free seats you stand for up to six hours. Even if you get a seat you can & will be turfed out of it if someone with a reservation for that seat gets on at subsequent station. This actually happened to me on an ICE train from Friedrichshafen to Koblenz, with the best will in the world its very hard if you have no common language. Luckily for me a very nice German lady who spoke excellent English jumped in to help me understand what the problem was & helped me find another seat..Apparently mid October can be very busy around the weekends on DB. I booked all long distance trains after that and indeed I had to push somebody out of my reserved seat on one of them.
Hope this helps, please ask any other questions you have and have a great trip!