Certainly it would have been preferable if someone could have caught these discrepancies; I can agree with that. I just don't know how realistic it is that that's always going to happen. Even the Berman era spinoffs had numerous discrepancies, and those shows were so slavishly faithful to each other that they eventually became recycled and boring.
Meyer's DVD commentary (the solo one) is a very interesting listen, in fact one of the best movie commentaries I've ever heard even just from a DVD commentary (forget about ST) perspective. And he does address the issue with Chekov, citing an instance in which Sir Conan Doyle similarly apologized for similar discrepancies between his Sherlock Holmes stories.
Back to ST, I find that every incarnation of it disagrees with every other on some level. The podcasters over at Trekmovie.com describe it as having to put on a different pair of glasses for every ST, and I have to say I've always agreed with that (though I couldn't have verbalized it as well as they did). I think it's the right attitude. Because what works in the Bermanverse, for example, doesn't necessarily agree with (to count them all off) what worked in the NBCverse, the Bennett-Meyerverse, the (god forbid) Shatnerverse, the Abramsverse or now the CBSverse... each with its own re-interpretation of what details were important to canon moving forward.
I would say it's more understandable than "lazy". Bennett and Nimoy probably both thought they remembered it as 20 years (as did perhaps many fans at the time, even ones who on any other day of the week might recall having seen 'The Menagerie').
It's infuriating to me just how quick a lot of fans are to use terms such as "lazy" when describing the creative efforts of people who write for, produce or direct their beloved franchises. It's already established that Bennett watched all 79 eps before producing The Wrath of Khan; he could even have stopped at 30 and that would be more than most producers taking on his job would have done. He actually did his research, and he still missed this one two years later. It happened. I would have missed it too most likely, even with most of 'The Menagerie's saved inside my head.
Yeah, some of it is minor, like the 20 year old reference. Chekov, I never got to hung up about that one because it's easy to imagine him being on the Enterprise before being assigned to the bridge, and some of the novels have even gone with that including Bennett's Ex Machina and Cox's 3rd Khan book. Cox even found a way to explain how they basically lost a planet (I have to admit that did bother me a bit in TWOK---I mean, how can you lose an entire planet, esp. one close enough to see it explode as Khan did--but his explanation did make some sense.
I'd probably hesitate to use the word lazy though, I mean unless it's something blatantly obvious. I'm usually pretty reasonable about minor errors. I sometimes get hung up with consistencies in production design, like the redesigned Klingons in the Abramsverse and Discovery, or the engine rooms in the Abramsverse movies that look more like they should be bottling water then powering a warp capable ship (though it was better in STID). But storywise, as long as things are reasonably consistent I don't sweat the small stuff too much.