A few responses - for some reason the OEBB site is still giving me trouble, even when I use different dates closer to now it keeps returning "ticket unavailable" when i enter Salzburg as a starting point and Melk as a destination. I tried doing it on my (standard) ancient computer (a 2011 iMac running macOS Mojave 10.14.6, which I know is not supported by a LOT of more recent "apps") and on my newer (2020 iMac running macOS Sequoia 15.4.1). That is where I managed to connect ONCE (out of about 10 attempts) and got the $ 400 Euro each cost (for second class) on an available train on OEBB.
I did check out the Westbahn site and that worked fine on the newer iMac (but not on the old one). The fares and schedule seemed quite reasonable from Salzburg to St. Polten or Amstatten, but they apparently don't serve Melk, so I'll probably still have to use OEBB for the St. Polten or Amstatten to Melk leg of the journey.
AND - we are planning to do quite a bit of driving before dropping off the car - we plan to visit both Southern Bavaria AND the Alpe di Suisi area in the Tirol, as well as Innsbruk, Salzburg and various other places. The reason for flying into Munich originally was to visit the Oktoberfest (we went to the Volksfest in Stuttgart a few years ago with our grandchildren, and it was GREAT). But that all changed AFTER the plane tickets were bought and paid for when my wife saw a video by one of Rick's staff which mentioned the "vomit hill" at Oktoberfest, which is apparently a place people are sent to relieve their excesses. She was so put off by that, and the cost of changing plane tickets from Munich to Vienna did the same for me, so we decided to drive on the earliest legs of the trip, and only take the train from Melk to Vienna. Dropping off the car in Salzburg is a piece of cake - and the extra fee Sixt charges to do that is nominal.
But with difficulty I'm having with OEBB taking the train from Salzburg to Melk and then on to Vienna is my difficulty. I guess I'll keep trying to figure out that OEBB Scotty app. Maybe I ought to try on my iPhone - perhaps they're not fond of iMac in that part of the world (we have no windows devices in this house - no offense to Seattle in general or Bill Gates in particular, but I first abandoned windows in my office, years ago, because of all the security issues, and then later at home - never a virus or so much as a security sneeze since we went entirely Apple there and at home)
In the meantime I appreciate the suggestion that we drop the rental car off in (or near) Salzburg and then take the train to Melk and on to Vienna. And so I keep up the struggle with OEBB which for me (a retired accountant, but nowhere near as tech savvy as I used to be) is becoming a definite problem. My wife and I have travelled independently for many, many years but I may have to engage a travel agent or tour group if this keeps up.
Thank you, though, for the replies!