A related question: is one lesson enough to enable safe subsequent practice on one's own which, in turn, can make that movement sufficiently instinctive?

No, but once you've seen what you need to practice, it can be done safely on your own on a carpet or grass (I would not recommend practicing on a hard surface; not that it won't work, but you'd get very sore doing it repeatedly).

Additionally, once you've had it shown to you in person, you can probably rely on YouTube if you need a refresher. I would not recommend skipping the actual lesson in favor of YouTube, however. It's best to have someone watching to see if you're doing the exercises correctly at least once.

No. Ive had fall technique trigger in accidents I did not see coming - including literally having a ladder yanked out from under me, but that was after over 3 years of 2-3 times week judo and jujitsu. Martial arts, however do have one major selling point. As exercise goes, a good dojo is simply a lot of fun. This makes it much, much easier to keep up than a gym subscription. Milage may vary for people who do not enjoy being tossed around as much as I do. I do emphasize "good". Quality of teaching and social atmosphere vary very widely. If you want to do this, make sure whereever you go has a tone you are comfortable with, and that their teachings has some sort of reality benchmark related to why you go. If you want to do it as a sport, try to find a dojo that has people in it that win contests. If you want selfdefence, the one the police use for qualifying training ought to be good (.. if your local police force is at all competent) but taking up parkour is probably more useful.