What does “Somebody is too much in one’s head” mean in contrast to “Somebody is in over one’s head”?

I think it amounts to meaning "too cerebral"—that is, too prone to considering multiple interpretations of data and the various pros and cons of possible reactions to them, without getting from that stage to the concrete response. Another way to express this criticism would be "too much thought, not enough action." It can also mean not giving due consideration to what other people are thinking or how they are likely to act—a problematic quality in a politician or strategist.

In contrast, "in over one's head" means "in a role or position that one is not competent (either by native ability or training) to perform satisfactorily."

“Someone is too much in his head” means that the person is over thinking things and probably should accept the simple answer/solution. The US military has an acronym K.I.S.S. which stands for "keep it simple stupid" as in don't make things more difficult than they need to be.

The previous answer for "in over one's head" is already well stated as; "in a role or position that one is not competent (either by native ability or training) to perform satisfactorily."