3
You are a victim of some misfortune, but you can also fall victim to misfortune (no article).
Idiomatically those two forms are fairly universally recognised and observed, but the idiomatic preference for "to become" is less well-established, with both become a victim of disease and become victim to disease being perfectly acceptable to most native speakers....
2
The preposition should be based on what type of mentorship or which aspect you wish to highlight. As such: for a field of study, "in" may be appropriate; a goal or personal/professional development target, or specific timespan would fit with "for"; "at" emphasizes the location (employer, institution, etc) over the subject of mentoring.
I might suggest ...
1
He is my mentor at the community college.
He is my mentor in mathematics.
He is my mentor from 10 to 12 this morning.
1
It is almost certain that the ambiguity:
We integrated A, B, and {C from The Cancer Genome Atlas (TCGA)}
We integrated {A, B, and C} from The Cancer Genome Atlas (TCGA)
will be resolved by the context when we know the origins of A, B and C
1
"Passives without an agent
Passive structures without an agent are very common. We use these structures when an agent is not important, or is unknown or obvious:
All applications must be received before 31 July.
The data was analysed and the results have just been published.
I walked to work. The car’s being repaired."
Cambridge Dictionary, passive ...
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