Sure, Kirk could have come back and extracted the seven corpses.
The fundamental fact of the matter is that humans cannot survive in space, or in ballistic flight. In contrast, humans can survive on the surface of a Class M planet for an indeterminate period of time, that is, a period not subject to countdowns of any rational sort. For a week, a human could survive on nothing but water licked off moist rocks, in bountiful supply at the crash site!
The only factor really threatening the survival of the castaways on the planet was a bunch of cavemen. But why should the cavemen be a threat? There was no evidence of them being organized or united. Unlike the savage hordes of Cloud William, they would have no cultural animosity towards the exotic arrivals. If they were in any way humanlike, they would have no concept of armies at this stage of cultural development, no experience in campaigns, and no interest in territorial squabbles. Our heroes weren't sitting on a coveted local resource - they were occupying wasteland, the dwellers of which would be sorry outcasts indeed by local standards.
The response of people like this to a deadly threat would in all likelihood have been a cautious retreat. But even a full-blown suicide assault would have done nothing but eradicated the threat posed by the very small mountain tribe eking out a living there. The key would be to stop sending human sacrifices to the locals, and to set up a camp of civilized people.
Spock was acting alarmingly irrationally here, perhaps due to his human half overruling his Vulcan one, perhaps out of misguided (and misunderstood) concern for his human companions' feelings. Unless his aim indeed was a swift, fiery death for everybody, as opposed to the assured survival of some until inevitable rescue. But he himself claimed he would not be that egalitarian in his decisions of life and death.