This would explain why some people recommend starting sentences with "I think..." etc. to reduce conflicts.

In a model-sharing mode that does not make much sense. Sentences "I think X" and "X" are equivalent.

I think it does make sense, even in model-sharing mode. "I think" has a modal function; modal expressions communicate something about your degree of certainty in what you're saying, and so does leaving them out. The general pattern is that flat statements without modal qualifiers are interpreted as being spoken with great/absolute confidence.

I also question the wisdom of dividing interpersonal communication into separate "listener-handling" and "model-sharing" modes. Sharing anything that might reasonably be expected to have an impact on other people's models is only not "listener-handling" if we discount "potentially changing people's models" as a way of "handling" them. Which doesn't seem to make a lot of sense to me.