I have concluded that I am trying to understand two, and perhaps three separate phenomenon about Nice People, which causes it all to be muddled. I will regroup and try again. In the meantime, a few things occurred to me today.
Jumping off from terri's Southern Ladies, who sorta fit my description but don't quite, I went looking for some other equivalent. The southern ladies who drop racist explosions in the cloakroom might think of themselves as nice people and thus are similar. Yet I think if one told them they weren't being nice they would acknowledge that, however many excuses and ignorant rationales they put forward. They would agree these were insulting things to say about black people - just deserved. My Nice People do not understand what you are saying when you challenge them. They are so focused on who they are being nice to and defending that they do not even see that this constitutes and attack on someone else.
One analogy that occurred to me is those folks who are always concerned with What Is Going On In The Schools, and finding covert satanic influences or softening the kiddies up for impending dictatorship. These days it's the belief that Common Core is not merely misguided education, but some active attempt to inculcated dangerous values in children. They start with some very real complaints and concerns, but lead you into this constricted maze of reasoning soon enough. Some go so far as to forbid CS Lewis and Tolkien because of MAGIC!!! and put forth Medieval rather than biblical theology in support of this. As no one is really much of a fundamentalist anymore, not even the cults, this oversensitivity is something of a positional good of more-Bible-believing-than-thou. (The link describes PC as a positional good, but I think it applies to fundies quite nicely. Useful concept.) At some point you start shaking your head and say "Do you realise you are not just calling these other people wrong and thoughtless, but satanic and evil? Do you really think that?" And the answer is not an embarrassed evasion but an oblivious one: Satan uses many methods, children are especially vulnerable, this world is headed toward catastrophe...
You just can't get them to the point of what terrible things they are saying about others, and do they mean that?
My Nice People are more like that. There is a competitive niceness that drove them into ministry or social work.
I may sort all this out in the next few weeks. Maybe not.