Personally, I'm all against Probert's design being the one for the E-C, but I have nothing against it being the definite Ambassador of which the Horatio would be the only known onscreen (if posthumous) representative...
...Except that it's a bit too complicated even to my tastes!
"Too complicated"? Surely, you must be kidding!
(funny, my guess would be the polar opposite, Probert's design is E-C and everything else Ambassador Class).
Now, all those Excelsior
prototypes look rather archaic and pre-TOS to me and I can't get one thought out of my head.
Remember that the Tauntaun mammal in the second SW film began as a dezert lizard, because nobody had told the production team on what planet they would need it for?
IIRC the production of ST III was kept a secret. I wonder (now that's a question for ILM) how much ILM knew about the premise of the film when they started designing starship proposals. Spock had died at the end of ST II, so inevitably "The Search for Spock" must have sounded as if Paramount Pictures wanted to shoot a prequel!
Not only would this explain the distinctive archaic pre-TOS look (IMHO) of the Excelsior
prototypes, but also the rather drastic design change that resulted in the USS Excelsior
as seen on screen. I have no problems imagining Leonard Nimoy saying "Sorry guys for keeping you in the dark, but we are shooting a sequel, not a prequel".
Apparently one design study wasn't altered - that of the USS Grissom