I made my video as I see it on the pro's and con's of leaning back out of range from Israel Adesanya at UFC 243. Generally it is used by elite level kickboxers as a head kick defense as much as anything. A lot of old school boxing trainers hate that movement style because it leaves you susceptible to guys who double and triple up on their punches. What do you think about it? Is leaning back a valid strategy. I guess you can say for self defense you need to be more worried about defending yourself against punches as it is unlikely you are going to come across a guy who is going to try a headkick with their shin in a self defense scenario.
There is a time and place for most all techniques. Likewise, most techniques have a counter that can be taken advantage of by your opponent. The more tools in your arsenal, the better you can be prepared for different situations. For the most part, I don't think of a technique as good or bad. I think of it as optimal or not the best choice for a particular situation. (There are a couple of things that I think are just plain dumb overall, though.) For self defense, a big weakness is that if you break your posture too badly, you are susceptible to being pushed on the ground or into say - moving cars. it isn't only punches and kicks one needs to worry about.
Mayweather. Yes. If all you do is snap back into position, the other guy will time it for the 2nd and 3rd punches - exactly the danger you mentioned. Very bad. But if you can mix it with some other movement (head weaving, footwork, hands engaged), that risk is ameliorated.
Leaning back when it's mixed with other head movement is a good defensive technique. Although there are counters to it. It's a great style to watch. There's a guy from some sect of the JKD community who has a system called the Ghost system iirc. I've always wanted to go to a seminar. My instructor says the style favours my body type.
You already covered staying over the leg. It's all about where your shoulders are in relation to your knees (presuming you're keeping your chin down). It's also about rotation, as you can see in the clips you posted. The Anderson Silva clip illustrates this perfectly: the first couple he rotates to load up one leg, but he falls when he tries to go straight back and his shoulders go past his knees. For me it's a last-ditch technique, unless I'm just playing in sparring. It's good to be able to do, but not a go-to, especially for self-defence. It can leave you wide open for leg takedowns, and the risk of losing your balance isn't worth it. It's not a good idea to go in intending to use it, but it is a great skill to have in a pinch when the only alternative is eating a strike.