1. “Tricia musthave noticed the pale look on my face and the loss of liveliness in my entire being that led her to question my well-being.”
2. “Tricia musthad noticed the pale look on my face and the loss of liveliness in my entire being that led her to question my well-being.”
In sentence 1
The sentence uses “have”, not because it is about a living person, but because the modal auxiliary verb “must” is used before the main verb in the present perfect tense, “have noticed”. The base form of a verb is used after a modal auxiliary, and the base form of “has”, “had” and “have” is “have”.
If you were to rewrite the first part of the above sentence without using “must”,
you have to write
“Tricia has noticed the pale look on my face.”,
using the present perfect tense “has noticed” ,
with the singular verb “has”, to agree with the singular subject “Tricia”.
There is no ban in English on using the past tense when referring to a living person! You can say: “My mother loved ice cream when she was a child.” even if your mother is still alive.