Well, however easy it is to sprain an ankle or break a hip, Shatner did the whole Kirk vs Air Force dudes fight in Tomorrow Is Yesterday, complete with a handful of stunt guys, punches, falls, jumps and swings off the doorsill. They even tackled him and slammed him to the floor. Made the brawl that much more exciting, and it the primary reason for my wondering why he didn't do more simple fights.
Again, consider the environment. This wasn't the bridge. This was a simpler, more open set -- just a floor, walls, a few shelf units in one corner, and a bin in another corner. It's not as tricky an environment as the bridge set where you have the two-level floor and the railing and the helm console to worry about. Simplicity or complexity isn't just about the performers' moves, it's about the circumstances they're in. It's safer to do a fight scene on a level surface than an irregular one, because there's less risk of your footing going awry, or of your head hitting some protruding edge if you fall down.
It's also a smaller set, requiring the camera to be closer to the performers, so that doubles would be more easily spotted. Even in the "Operation: Annihilate" scene we evaluated earlier, they cut to the regular actors for the close-in portions of the fight scene, and only used doubles in the long shots. Here, it was all medium or close shots, so doubling wasn't really an option.
As for whether or not the stunt guy in Space Seed or Court Martial was noticeable before HD, damn straight he was. When Kirk does the running kick to Khan, the guy looked totally different; different body type, different hair, totally different face. Khan also got like 6 inches shorter.
That depends on the TV. For the kind of sets people grew up watching TOS reruns on in the '80s or '90s, sure. For an expensive 24-inch color set with good reception in the '60s, probably. But for the dinky black-and-white set I grew up watching TOS on in the '70s, or with the kind of static or image distortion I often had to deal with when the antennae and knobs weren't adjusted just right? I doubt I could've told the difference. We're spoiled by the TV sets we have today. Not many people remember what it was like in the days when you had to wrestle your TV into submission to get a halfway decent picture out of it, and then just hope it would hold that way for the duration of the show.
(Good grief, I sound like an old fogey.)