I know what you're saying. And I agree you can't write off all of what they do and say as "irrational thinking" and that some people try to do that. I believe you do need to hear what they are saying... even when it's completely off the wall.
In some cases, I am sure you're right that the mind is made up, or at least they believe it is.
But I know, in my sitch, the man I've lived with for 15 years was not at all the man my H has been during most of our separation.
You see, my H was a complete mess when he moved out. I can't even begin to recount the ways in which his behavior scared the he!! out of me. There was nothing rational or clear about what he was saying or doing at that time.
I think one telling factor about whether there is more going on, whether its MLC or other factors, is to look at the rest of their lives and relationships, their job, how they are functioning in their lives other than the S.