Hi Ray: I'm convinced that if Mario had appeared in
The Student Prince, it would have eclipsed
The Great Caruso---artistically and probably commercially as well. More importantly, Mario would not have begun the downward spiral that shortened his life so drastically.
If we look at photos of him in the first half of 1952, we see a man in the best physical condition of his life (despite the fact that he'd lost weight far too quickly while filming Because You're Mine). He looks on top of the world---and he had every reason to be, as he approached the recording sessions for The Student Prince. It's heartbreaking to contemplate what might have been. Just think: if Louis B. Mayer hadn't been forced out of his position---if he'd just remained as Studio Head for another year or two---Mario would never have walked out on MGM. Mayer would have found a way to keep him happy.
Mario's contract wasn't due to expire until 1954, so we would have had at least one more film---probably two---at MGM after The Student Prince. (The Vagabond King, perhaps? It had certainly been talked about enough.)
Yes, I do think there would have been a purely RCA version of The Student Prince if Mario had remained at MGM. Perhaps Gale Sherwood, who (after all) did attempt to record "Summertime in Heidelberg" with Mario in December 1953, would have been the soprano. I can imagine it being a more operatic version than the MGM recording, and with different arrangements. And yes: probably with the original lyrics and perhaps the two extra songs he recorded in 1959 (although "Just We Two" was never the Prince's song). I don't think for a minute that RCA would have dropped the three Brodszky-Webster songs, though. In fact, Mario attempted to record "I'll Walk with God" at that aborted December 1953 session when it still seemed that there would be a separate RCA non-soundtrack album.
So many tantalising "what if"s!
Derek