Sorry, but it's bonkers to suggest ILM were designing "retro" ships to make fans think they were filming a prequel. The Excelsior at least was always
intended to be a bigger, faster, sleeker, longer, more imposing take on the Enterprise. Nilo Rodis recalled:
The "Excelsior" had to be brilliant, and it had to steal thunder from the Enterprise.
Bill George took the view of:
OK, what would the "Enterprise" look like if the Japanese designed it?" That was the basis of what I came up with for the "Excelsior".
It's a movie. Everything's budgeted. You don't waste money on some elaborate subterfuge if you have no intention of using the finished work, so the idea of decoy TOS-like study models is incredibly far-fetched (and as Dukhat says, they don't look remotely retro anyway). Especially given that in the pre-internet days, you'd have to be a very dedicated fan to even hear about study models. And frankly, it doesn't matter if a handful of geeks find out the plot of the story, when you're ultimately going to call the film "The Search for Spock". Bit of a give-away there I think.
There's a decent summary of the design process here