We have a long (and continuing) history of comments turning into arguments. I gave you some (IMO) good advice in those now-deleted arguments... because I have lots of experience doing it wrong over that history. It sounds like KRyan also gave some good advice, and I know he got that experience the hard way too. So that's the context.
It sounds like your constructive comment got nuked along with the rest. Given the comment-argument history on RPG.SE, it probably simply got caught in the cleanup rather than being specifically judged and found wanting.
On your specific questions:
Is there a good place for asking specific moderators specific questions?
Meta is the right place for this, so you brought this to the right place.
Should I have taken this to meta with my specific comment in the first place?
Maybe? But probably not. Meta posts of that kind tend to be more about issues with questions, such as their validity or topicality or how to handle them in some other way. Suggestions for improving a question don't tend to come here, since it's not a crisis either major or minor—questions that could use improvement either get improved or they don't, and the votes follow, so there's rarely a productive point in blowing that up into a full meta conversation.
If my comments really are that bad, how can I get directed feedback that will make me a better member of the community? Again, I'm legitimately trying to understand what was wrong with my comment, not just raging against the mod-machine.
Comment deletion is pretty common on SEs and here in particular. They're deliberately ephemeral and expendable, so that SE doesn't become just another discussion board. That has implications for both parts of this sub-question: firstly, since the bar for deleting a comment is so very low (sometimes as low as "it's been a week, time to clean up"), deletion isn't intended as a value statement about the comments, so your take-away shouldn't be that they're "really ... that bad".
Secondly, that they were deleted for mere tidying reasons, not "moderate bad content" reasons (like how deletion is done on a forum), so they weren't judged as quite "that bad" in the first place. They just weren't being constructive (leaving aside that one comment); a comment to survive a cleanup means that it has overwhelming value in being left, and everything below that very high standard goes in the bin. This is because of the philosophy that either comments get incorporated into what really matters here—the posts—or they must not have been useful. Rarely, a comment will survive for other reasons, such as pointing out something that doesn't really belong in the answer, but should stay for information reasons.
If there's no way to get feedback on that sort of thing, should I just hang up my SE hat? I obviously don't fit the mold yet, and my options for improving seem to be dwindling. I was hoping it was a philosophical goal of SE to build up content providers, but maybe it's only to find the ones that are already a match?
Actually, your conduct during that comment exchange was exceedingly commendable. You made an initial few comments in a way that was not constructive (according to the admittedly arcane dynamics of this site), but when you got brusque comments from me and others about that meta-concern you actively engaged with curiosity rather than reacted with offended defensiveness. That is something that many of our top users can't manage to do except on their best days. Most of us can't keep our egos in check if comments become argumentative, yet you handled the disagreement with integrity and turned it into the pursuit of knowledge.
Your bringing this to meta in a non-confrontational way is also commendable. These things say to me that you are actually very well suited to the site. Someone who can handle disagreement with an even temper is hardly necessary to do well here (as I am living evidence of), but would give you a distinct edge in having constructive conversations about questions and answers that need a little bit of help and TLC to become their best. That you managed to demonstrate that within the span of a single comment argument means that I, personally, have no worries that you will run into similar (but I say again, minor) friction in comments any time soon—in fact, I suspect that your comments in the future will contain a degree of diplomacy I can learn from!
So there is no need to hang up your hat. You've been getting votes and accepts on answers, and that's really the most important measure.
Comments... well, comments are a thing that we can get personally invested in, but at the end of the day isn't what RPG.SE is here for, so that investment is sometimes best seen as a distraction. It's taken me a while to come to it, but now when a comment string that I'm invested in gets deleted, I shrug and remind myself that its not what we're here for, and that "making" people "fix" their errant answers isn't part of the core system of SE that makes it so valuable.
When I remind myself of that, it helps me let go of the comments and get back to the voting and answering, so that I'm voting up good answers and providing answers to be voted on. I will still often feel like an answer can be improved with changes... but, eh, there are other answers, and the votes will tell, so that helps let sleeping arguments lie.