Neither the uniforms, nor the general tone, seemed "more militaristic" to me, going from TMP to TWOK. I'm kind of inclined to view the TMP-era uniforms as simply having been discarded after a few years as impractical and/or uncomfortable, and the TWOK-era uniforms enduring almost to the TNG era (albeit losing the turtlenecks at some point) because they proved to be both practical and comfortable.
Seriously? I'd say it's the other way around. The TMP uniforms look quite practical and comfortable. They come in a wide range of variants for every possible use. They have one-piece and two-piece versions, long-sleeved and short-sleeved versions, field jackets and other variations as needed -- immensely practical and believable as everyday duty wear. The TWOK uniforms were Horatio Hornblower cosplay that might be plausible as full-dress uniforms but look far too heavy and cumbersome to be practical as everyday fatigues. Ditch the heavy jackets and just go with the turtlenecks, like the pilot-era uniforms, and you might have something. But as they are, those uniforms don't look at all comfortable.
The novelization of Star Trek VI states that the reason for increased Federation/Klingon tensions is that the Organians had seemingly disappeared.
Yes, of course I'm aware that multiple tie-ins have concocted various versions of "the Organians went away," as I already said. My point is that I think that's misunderstanding what was implicit in "Errand of Mercy" itself. They don't need to go away, because they made it crystal-clear that they don't want to interact with us at all if they can possibly avoid it. As long as the Federation and Klingons left Organia itself unmolested, the Organians wouldn't intervene in anything they did. After all, TV episodes back then had to be self-contained and standalone, so Gene Coon had to build in a reason why we shouldn't expect to see the Organians ever again. The explanation was already there without the need to contrive "mysterious disappearances" or whatever.