Not sure if this fits one of the reasons on your list, but I maintain a (short) mental list of LWers I never reply to, regardless of what they write, because a number of prior conversations with them were invariably futile for one or more of the reasons you describe, and my estimate of any future conversation going any better is very low.
I am guessing that other people might have similar lists, and if so, I am probably on one or more of those. I conjecture that this is the case from the downvoting pattern on some of my instrumentalism-related comments.
I have on occasion explicitly mentioned when I add a person to such a list, but this is usually detrimental, so I stopped doing so. This is not to say that I always downvote their comments, often it is quite the opposite, I just don't ever engage them.
Oh, and once or twice I noticed an extreme change in attitude from a regular and started replying to them again.
Just wanted to add that at some point I thought of making my ignore list public, together with the reasons and examples of why I add someone to it, in case someone would like to clear the possible misunderstanding, but then realized that such an action is virtually guaranteed to backfire. It would probably be misunderstood as inappropriate status signaling and piss off not only those on the list, but many others as well, and defeat the purpose completely. So I took the middle road of disclosing the existence of my ignore list in an appropriate thread.
That said, if anyone cares enough, I will certainly reply to a PM from anyone, regardless of whether I reply to them in public.
I often feel guilty for ignoring other people's comments or questions, and frustrated when other people seem to be ignoring me. If I can't indicate to someone exactly why I'm not answering, or can't receive such an indication myself, I can at least help my future selves and others obtain a better probability distribution over such reasons. To that end, I'm listing all of the reasons I can think of for someone to not respond to a comment/question, to save the effort of regenerating these hypotheses from scratch each time and prevent the possibility of failing to consider the actual reason. Note that these are not meant to be mutually exclusive.
If I missed any reasons (that happen often enough to be worth including in this list), please give them in the comments. See also this related comment.