Usage of "must have" in past tenses

Must has no past tense. Instead we use the past tense of have to. That means your first example should read It had to have been 10 years since … and the second She had somehow to have made herself appear shorter … I don't pretend that either is ideal.

There is no alternative to "must have". All modals, without exception, are always followed by the bare infinitive. I could write several pages on why this might be so, but no one would be any the wiser. Suffice it to say that some things should simply be learned as properties of the language. Modal + Infinitive is one such instance.

In the particular case of "must", you might want to consider that in expressing an assumption the assumption itself is always formulated (if not expressed) in the present. "John had thought". Is a report on a past event about a prior event. Thus, past perfect.

"John must have thought" is an assumption formulated in the present about a past event. A perfect case for the present perfect, which serves to bridge the present and the past. One, in fact, is going one tense back, from present when the observation of what John must have been thinking was first articulated to past (when John in fact had been thinking something).

In this case, "must" has taken us to a meta-level of sorts: The observer is observing the observer.