grammaticality - Usage of "must have" in past tenses

So, I've checked Is "must" ever grammatical as a past tense verb?

and Past tense of "must" when meaning logical probability and I'm also almost confident that I cannot say "must had to." How can I say that?

For example, in these situations (I'm copying the examples from the wordreference forums, where they are still not answered):

But translating a story, which is already in the past tense, I came across the following problem: What if a text is already in the past, and you basically have to go one tense further back grammatically to express a surmise/conjecture about events even further back in the past than the past of the story. In other words, is there a past perfect form of "must have"?

Here's the context to make it more clear:

[…] He was walking down the street, mulling things in his head. Maria? Was it really possible that they would meet again here, in London? It must had been 10 years since they had last talked to each other […]

The trouble I'm having is that this doesn't sound quite right to me, but neither does it sound good (to me) if I substitute "must have" for "must had." In the context, it doesn't seem "past-tensey" enough, if you catch my drift. So, would the above sentence be grammatical or not?

A similar example is the following one:

[…] The truth was, Frank was not that much taller than Maria; she must somehow had made herself appear shorter […]

Should it be "must somehow have made herself appear shorter"? Again, would that be "past-tensey" enough?

Answer

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.