First, I'm greatly amused by the contradiction between your username and your bludgeoning insistence that other people tell you what they like/don't like/find of merit/don't find of merit regarding ecchi. If you - the only being that you're certain exists in the universe anyway - don't find merit in ecchi manga, then don't read ecchi manga. That seems pretty simple, really, and I'm not even quite sure why a solipsist would be concerned at all about what theoretical other people think or don't think about the subject.
Now - as long as I'm here, since you're asking (and I'll freely admit right off that I didn't wade through everything you've posted here, so I have no idea how this might or might not jibe with what you've posted beyond the OP):
Yes - plots sometimes, arguably often, suffer as a result of a focus on ecchi. I mostly don't care. If I'm in the mood for something ecchi, it's because I find sexual stimulation to be pleasurable, and that's what I'm looking for, so the primary measure of the merit of the manga is how well it serves that purpose. If the girls are hot, then it's done its job. The plot, what there might be of it, is secondary anyway. If, on the other hand, I'm looking for something with a plot, I read something with a plot. I don't see any conflict between those two things, just as I don't see any conflict between the fact that sometimes I crave pizza and other times I crave ice cream. When I crave pizza, there's pizza to be had - I'm certainly not going to waste my time complaining because this ice cream over here is ice cream instead of pizza - I'm just going to go get some pizza and save the ice cream for when I'm in the mood for ice cream.
Back to manga - most often, ecchi is pretty much the point, and what little plot there might be is just an excuse for the ecchi. That's usually fine (refer back to the pizza vs. ice cream bit). Sometimes the ecchi is particularly painfully contrived - to the point that I find the manga pretty much unreadable. If that's the case, I don't read it. Problem solved. And it's certainly true that there are a number of manga that would be better if they invested more time in the plot and less in the ecchi, which is particularly noticeable with titles (like HSDK) that transitioned from plot>ecchi to ecchi>plot. That's sort of the way it goes. It's disappointing, but the world's an imperfect place.
And, very rarely I'd say, there are manga that manage to include ecchi fairly seamlessly. Yes - that's not terribly common, but neither is a thing that combines the best parts of pizza and ice cream. I don't set my expectations that high, so I'm not disappointed when they're predictably not met, and pleasantly surprised when they are.
Enjoyable ecchi?
Probably the first best example I can think of is My Lovely Ghost Kana. Even beyond the (tasteful but relatively explicit) sex scenes, Kana is exceedingly easy on the eyes. It's also unquestionably one of the most brilliantly well-written manga I've ever read. Full stop. The ecchi isn't really central to the story, and is thus a bit distracting, but in spite of my culturally ingrained aversion to explicit depictions of sexual congress (and privately adopted and exceedingly common fascination with same), I neither mind them nor are inflamed by them. Ultimately they're just there, and they do serve some purpose in the story, since they highlight both the passion of their relationship and the realities of her situation.
A completely different example of a manga in which I found the ecchi wholly acceptable conceptually is Yomeiro-Choice. It's completely different, in that the ecchi is wholly egregious and there's often virtually no plot other than whatever might be necessary to provide some colorable justification for the ecchi. The difference is that the entire series is satirical, so (since you appear to be fond of TVTropes tropes) every one of those terribly transparent and shallow setups to cliched ecchi is lampshaded, so the transparency and shallowness becomes part of the point of the whole thing, and ultimately adds to the humor. Unless one doesn't appreciate satire, in which case it's much better avoided.
I personally like the way that Mizukami Satoshi handles ecchi. One certainly couldn't complain that Hoshi no Samidare or Spirit Circle suffer for any lack of plot complexity, yet they both take opportunities to slip in ecchi here and there, and I much appreciate it. I guess it's a bit egregious, but only because the real world doesn't offer panty shots quite as often. But it's really not much different from the real world - the people are busy doing all the things that they're doing and the story is advancing and we're getting ever more involved and hey blammo! there's a panty shot. Score. And the story just keeps on going.
Another, more standard ecchi series I don't mind much is Trinity Seven. An awful lot of it really is just an excuse to show the girls in various stages of undress, but it's actually a plot point, so it doesn't really suffer as a result. And honestly, if it wasn't for the ecchi, I don't think I'd bother reading it, since I'm just not that interested in yet another guy at magic school manga, even if the guy is notably more confident and powerful than the norm.
Beyond that, there's a goodly amount of ecchi on my follows list. Again, I happen to find sexual stimulation pleasurable and I don't begrudge myself that. I'm sure there are others that I think manage to do at least a reasonbly good job of balancing ecchi with plot so they don't end up too painfully shallow and contrived, but I'm equally sure that there are probably many more that really are painfully shallow and contrived and really are nothing more than feeble excuses for ecchi. I simply don't mind.