I'm guessing you've read Ed Miller's hand-reading book, since you specify 30% PP's etc.
To be honest I think you're trying to think too generally about something that's very situational. But you're also asking a question and then pretty much answering it yourself, so I'm not really sure what you want lol. But I guess I'll toss out some thoughts.
I'd say that one very common spot where someone's range is capped preflop is when they flat a 3bet. Say they open in MP and face a 3bet from the blinds. If they flat, most players won't have KK+ in their range. To try to answer your numbered questions though...
1. Against an "ultra-nit," this is probably a fine spot to do anything, but I'd prefer him also fold a fair amount of his overpairs to a x/r if that's the play I want to make.
2. Typically, you don't think about a capped range as being one that can improve its hand value very much. That's part of what makes it capped - draws aren't typically a part of a capped range. Capped usually just means "not that strong," and "not much potential to improve." Something like a PP-heavy range that consists of mostly 2nd pair hands is a very capped range.
3. You've pretty much identified a number of examples where opponents have capped ranges. And LAG's will by definition have wider ranges at every situation when betting. And usually this means they will be capped less often, since they will have more air, more semi-bluffs, and more disguised value hands. But in terms of identifying a capped range in general, you've already mentioned how - hand-read and notice that the range is semi-weak to semi-strong, and can't improve much.
4. Short answer to this is that you can't. If you are up against a solid hand-reader and your range is capped, he will likely know. A balanced range is inherently NOT capped, since a balanced range will always have some strong value hands, medium strength hands, and draws/air. But short of having a balanced range, you can't prevent someone from knowing you are capped if you are. Any strategy that prevents it would involve some type of balance, whether it's all the time or a long-term "changing gears" type strategy. But you can rest fairly easy, because you will find very few opponents who will be good at analyzing your range accurately, and even fewer who will analyze it AND exploit it.