Yeah, but as I recall the Guardian's playbacks were being somewhat randomly generated, (i.e. though cycling, not going in any real logical order)
I chalk that up to limitations on film editing and available stock footage. It was meant to symbolize the Guardian racing through Earth history, and that was just the best approximation of that idea that they could pull off visually.
Yes, she wasn't strictly "meaningless" in the altered timeline, but it still seems like that's pretty detailed information for a tricorder to pick-up over a scan of random points in Earth's history and various locations as they appear over the course of a few minutes. I mean, that tricorder would have had to have information on *everything* in order to do this.
Well, the episode depended on the premise that there were "currents" in history that led travelers toward key nexus points like Edith Keeler, which is the only way they were able to find McCoy. So perhaps the Guardian's playback also focused on those key nexus points, instead of just covering every moment of history indiscriminately.
This is sort of a conceit we have to accept when it comes to time-travel stories. Technically our heroes should be vulnerable to all kinds of things walking around so far in the past, things their bodies aren't going to be immune to because they're things they don't encounter in present-day. Vice-versa is true as well, they're going to expose contemporary inhabitants to the evolved/adapted diseases and viruses from the future. Things we're immune to due to the changes in technology, forcing the micro-organisms to change, further pushing our immune system to change they're going to bring that back to a time period that is hundreds or thousands of bacterial "generations" behind.
Yeah, disease is an issue that time-travel stories routinely ignore, except on rare occasions. Same with space-travel stories, although that may be less of an issue with alien biochemistries. (Although if you can eat an alien planet's food, it stands to reason that its microbes could eat you.)
So, a conceit we have to accept in time-travel stories is just that "somehow" these complications from food and disease aren't a problem.
Well, it's less fanciful than the conceit that time travel is even feasible, let alone the conceit that history can be overwritten.