We can only gauge what's possible off-panel/off-screen by what we see on-panel/on-screen, but that's beside the main point. Do you know of at least one occasion where Ranma just up and decides to learn some martial arts technique or style without being compelled by circumstance, or inspired by an example by someone or something other than himself? (Aside from some training trips, as I don't recall their exact purpose being given. And in some cases whose idea they were.) Something like: "You know what, I feel like learning something new. I wonder if I can find anything or anyone that will teach me something." I'm not saying he's incapable of it, but the general lack of actually seeing it in light of how often we see him learn new martial arts stuff elseways should mean that it's far more likely for him to not take the initiative (want to learn something new and make an effort to find it himself) when it comes to learning martial arts. Plus, if not for the circumstances surrounding many of the times when he learned something new, he probably wouldn't have bothered learning them at all. Just take away the challenge, the dilemma, the petty revenge, the injured pride/insulted ego, the necessity, et cetera and he wouldn't have cared about learning most of the techniques and styles we've seen him come across.
I don't recall a topic concerning how serious Ranma is as a martial artist (that's my memory for you), but I'd like to point out that there's a difference when someone's serious about martial arts and when they're serious about winning or think it's important to prove themselves. To me, Ranma is one or both of the latter two, with martial arts being a means to either one of those ends. Anyway, to more specifics.
I also disagree with you Ranma in my opinion would happily learn any thing about martial arts that he found out
I've neither seen him happy in all cases nor try to learn everything he's been exposed to (being that he's never seen learning it or using the technique or style). Even for things that are useful and/or impressive, like the toughening that results from learning the breaking point technique (there's no, "my kick feels like a baby's foot? Well, so does yours!"), the hidden weapons technique (he just pulls out things like everyone else can because it's a gag manga), or even the
legendary split-body technique that Konatsu shows off.
As for pressure points/weak spots as stated in another thread Ranma used them more than once. There was the time with Miss Hinako, but also Kuno during the battle in which Ranma hit a whole bunch of them faster than Nabiki could see in a very early volume, on the Dojo Destroyer when Shampoo put the fake cures on them, the attempt to hit Ryoga's sleep spot in the girls locker room, etc.
The weak spots appear to be basic knowledge that fighters are supposed to avoid hitting in an official match. As for the "sleep spot," that seems pretty handy yet I only recall Ranma trying to use it that one time, so it may not have been a sure thing (using a shiatsu technique) and may have simply been a focused blow toward a spot that can knock someone out. (I remember others attacking with two fingers like that, and it wasn't shiatsu. No specific time comes to mind, but I'm pretty sure that Ryoga did it a time or two. The koi rod story is probably a likely place.) As for Hinako, the circumstances required learning it, since he wasn't safe from her energy-draining technique either.
The going through Happosai stuff was (going by memory here so not sure) during the Martial arts dining story arc or the one with Lucky (also can be seen in Happosai's room trying to learn something from Happosai at other points like during the time Happosai was making a rejuvenation potion).
Ranma didn't go through Happosai's stuff during the martial arts dining story (he was stuck at the Chardin's mansion), and he didn't ask the others to do it. He only entered Happosai's room to fight Lucky. The youth potion wasn't for the sake of learning the youth potion, but to (supposedly) help Happosai, who had been trying to make it.
Most of the people such as Cologne or Happosai that could theoretically teach Ranma either don't want to or would only do so with strings attached that Ranma would not want to pay the price for (such as marry Shampoo or let himself be fondled).
In the case of Happosai, I can see Ranma refusing to ask simply because he wouldn't want to be associated with him (even though he's the heir). Cologne, on the other hand, is where you're wrong. Take the Phoenix pill, for instance. She didn't make it so he had to marry Shampoo in exchange for the Phoenix pill, but made it the option if he gave up on the fighting chance she gave him. There were absolutely no strings for the hiryu shoten ha. Nor were there any strings for any other time she got involved and offered her help (the Herb, dragon whisker, and Pink and Link stories come to mind). She may want him to marry Shampoo, but there's no evidence that she would ask him to marry Shampoo if he asked her to teach him something. I think she's smart enough to not go down the "you've got no other choice" route, because that would be the best way to ensure that Ranma would never find marrying Shampoo appealing. Oh, and she didn't try to force a love pill down Ranma's throat and make him look at Shampoo. I think that's telling enough.
And, really, there's no excuse to not learn something from Tofu. Considering that the only shiatsu Ranma's ever used may have only been used on Hinako (even though shiatsu appears to be very useful), I don't see the problem with him borrowing the same book that Kasumi had once borrowed.
It also appears that Ranma does improve his techniques pretty much constantly since when do we ever see him use a technique in the same way multiple times? It is far more likely like with the Hiryu Shoten Ha or the moko takabishi (double version) that he'll use a new variant on further showings rather than use it in the same way and we do see him use techniques that he had not displayed before and the training for them was not shown at all on screen (like the time he cut a perfect hole in Ryoga's house we never saw him train to do that). The thing is most of his training is not shown so it is mostly conjecture (we see very little of his training usually only snippets sometimes only a single frame off a page before it gets to the story since the training is not important to the storyline). He could have trained in many techniques that simply weren't shown because they were of limited use, excessively dangerous (Ranma does not fight to injure his opponents), other techniques were simply more practical (for example Ranma has not been in a situation where the breaking point was useful to him the one time I recall where it might be useful Ranma decided to cut a perfect hole in the wall rather than use it), etc. Whenever he trains, who's to say that it's for anything new maybe true but other options are equally valid after all who's to sat that he isn't training to learn new things or that some of the training trips mentioned but barely shown weren't to find new things?
You can't measure his interest in martial arts, or how seriously he takes it, if what we see him do is due to something he learned out of sight (unknowable time, reason and circumstances). We can only measure this by what is seen, and it's usually through necessity and circumstances not of his choosing. Even the changes we've seen him make to techniques that we've seen him learn were done spontaneously (with the possible exception of the double moko takabisha), not due to training on his own time and terms. And being a good martial artist doesn't necessarily make someone a serious martial artist. In much the same way that a smart student can get bored with their school work and lose enthusiasm/interest.
(And I think you mean that Ranma avoids maiming his opponents, seeing as injury in general is next to impossible to avoid when it comes down to physical blows. If Ranma went far enough to prevent injury, then he'd probably use shiatsu... but he doesn't.)