Good point, but they never left the ship. We're not beaten over the head with it, but nothing was really as it seemed since the probe mind-linked with them.
I've had a mind slip about this episode, as I had it fixed in my mind, for some reason, that the buoy was destroyed at the beginning, after Enterprise had received the warning. Doubly embarrassing as I just used that remembrance in another post.
I have to admit though, that in the many times I've actually viewed this one, your contention has never occurred to me. Taking the meaning that way certainly fits with the continuity of the episode as it ultimately plays out. Are you implying that the entire scenario that ostensibly takes place on the planet, is essentially of a moment's duration as the "landing party" and crew experience it? If not, I suppose the rest of the ship's complement has been influenced to think that Kirk et al. actually did beam down and are simply under the impression that they are waiting for them to reestablish contact, correct? Also, why does the buoy self-destruct? Would an explanation be that while it seemed to have let the ship approach the planet initially, it actually hadn't, and now that it's played its purpose, Enterprise is being allowed to proceed, but, if so, why does that necessitate it blowing up?
The interpretation does make sense in a number of ways that fills in continuity, especially obvious when the Melkotians invite Kirk and company to advance to the planet, which would plainly imply that they hadn't done so before. But at the same time, it would seem to materially change the thrust of the plot. How was the landing party ever really in danger if they never faced the supposed existential threat that was seemingly imposed on them, and in fact were just experiencing the illusion as a type of forced fantasy or daydream, while they were actually safe aboard Enterprise? Interestingly, when seemingly returned to the ship, no one (not even Spock) explicitly surmises that this, in fact, is what just happened to them, instead apparently further validating their presence on the planet, however manipulated the circumstances, by merely referencing a rationale for Chekov's survival that doesn't question at all the premise as they experienced it and the audience viewed it. Accepting this view of events, also seems to change the character of Spock's realization, though not necessarily diminishing its importance.
While I did get a significant plot element out of place and drew an incorrect partial conclusion because of it in the post that I mentioned above, the overall sense that I made of the episode would seem to materially agree with the meaning that I think one would have to surmise from what you suggest. That is, the challenge being presented to the landing party was really a test, one that the Melkotians perhaps used as a standard procedure, to value the worthiness of an alien civilization's stated intention to make first contact. So all the machinations that the landing party worked through were in effect, part of a thought experiment that, depending on how it was reasoned out and its conclusions implemented, would determine the value (moral,ethical, or otherwise) that the Melkotians would assign to this alien culture and form the basis of their decision to allow contact to take place.
It would seem that in your perception of how the events happened, a response that was deemed a failed one, wouldn't of necessity, result in anyone's death, but perhaps simply in a steadfast refusal of the Melkotians to engage any further with the visitors, although that determination could ultimately be enforced by a further telepathic contact that would be designed to manipulate the visitors into some kind of self-destruction. The proposition that I advanced might have advanced to the same conclusion, the difference being that in accepting that, as in this instance, the landing party was actually on the planet,
their fate in failing to comprehend the real parameters of the situation or explicitly articulating a violent nature, would have been sealed there and then.
Perhaps this understanding is a fairly common taken one that I've just never encountered, but seems more or less obvious to many serious observers of the episode. It seems to me, that it substantively changes its character, by introducing a more nuanced and multi-faceted aspect to the presentation.
I would conclude by asking if, in clearly having this view of how the experience takes place, you (and others) would tend to agree that the purpose of the exercise is to enable the Melkotians to test and judge, as has been the theme in a number of other TOS episodes, the innate qualities of a culture that appears, at least superficially to be inferior, as to their suitability for genuine encounter, rather than as the stated punishment for a nominal offense that has been committed?