Matt Jefferies definitely created a sketch that had that scale on it. And I'm sure that his originally-intended number was ~950'.
However, remember that he was working for a 1960s TV show, on a tight schedule and budget, and was creating a ton of content on a daily basis. The fact that we got as much detail and accuracy as we did is impressive, no question.
But to claim that his work, on paper, is "inviolable" is sort of silly. He, himself, never felt that way about his own work (as indicated by his frequent revisiting of concepts, not to mention his sketches for updating the E later on).
Nobody can speak for M.J. but I feel confident that his main concern was coming up with a ship that looked good and which worked. I sincerely doubt that the man would have insisted that 947' (a figure derived by review of a diagram never intended for close scrutiny, and expected to be seen only on 1960s TV sets) was more important than, say, making the sets seen on-screen match up with the physical exteriors seen on-screen (even though, as we know, there are myriad instances where this simply can't be done anyway, due to production shortcuts... the TMP rec deck, anyone?)
Given infinite time and resources, I have no doubt that M.J. would have continued to evolve his design. Would he have come up with any of our specific solutions? Maybe in a few cases, but most likely he'd have come up with something which differs, at least in some measure, from anything any of us have envisioned as of yet. That's how creativity works, after all!
947' is NOT a "firm stake in the sand." It's just a number that was tossed out as a "good enough" estimate for the purposes of a TV show, and which was never mentioned on-screen or seen on-screen.
NO, THE DIAGRAM ABOVE WAS NEVER SEEN ON-SCREEN CLEARLY ENOUGH TO DERIVE A FIRM LENGTH. And it was never intended to be seen in the detail we now do get to see it, anyway. It falls into the same general category as the "insurance remaining" indicator on the TNG medical diagnostic reader display, or the little "nomads" and "ducks" and so forth seen in TNG-era diagrams, and so forth.
I have no problem at all with people being dedicated to keeping the 947' length for their own purposes. My only problem comes up when someone insists that things we ACTUALLY SAW, ON-SCREEN, must be rejected in favor of a number which was never established to the audience.
What is canon, is what was seen on-screen... well, except when it isn't. We don't HAVE to accept that the Enterprise had nacelle domes one second, and didn't have them the next second. We don't have to accept that the ship's gymnasium is EXACTLY like the ship's engine room. We don't have to accept that the ship's landing bay has a bare, featureless wall. We don't have to accept that the Galileo is a TARDIS.
There's nothing wrong with the "what would the thermians do?" approach... and it's actually quite fun to watch. As long as nobody starts claiming that this is the "right" solution, and every other solution is wrong.
That, unfortunately, is what some people have done with the 947' number. It's silly, really... especially when it means that the sets won't fit without major revision. Rejecting sets (which saw more screen time than the ship exterior ever did, much less a diagram seen once, and then only in a very blurry fashion) seems... well, like misplaced priorities, that's all.
The Thermians would likely end up assuming that a "foot" is larger than 12" and then figure out how everything works. (After all, the term "foot" as a unit of length was never established on-screen!) Or they might have concluded that the entire crew was under 4'6". Either way... that's the only real way to reconcile the sets and the ship's exterior without significant compromise to either.