Calm yourself sister! I believe you may be ascribing more faults to Mary than she actually has when you call her a "black-hearted trollop" and a "woman of small morals and questionable virtue."

Trollop has two meanings: 1. 'untidy/slatternly' - the sense which JA uses in ch.38 - "trollopy-looking maid-servant". I do not think this meaning applies to Mary.
2. 'Shameless/promiscuous'. Is this the sense you meant?

I don't dispute that Mary is "of small morals", but "questionable virtue" seems to imply s*xual misbehaviour.

I can't find evidence which suggests that Mary is guilty of any s*xual impropriety. She fails to understand the moral wrong of adultery, but this is not the same as being guilty herself.

Speaking with Henry about his marriage in ch.30 Mary says:
"... even when you ceased to love, she would yet find in you the liberality and good–breeding of a gentleman.”

I take this to mean that she thinks male infidelity is very likely after the first hormonally driven rush of "love" has worn off, but that Henry would be discreet about it and not make his wife unhappy, unlike the Admiral.

But Mary has no intention of behaving in such a manner herself. In ch.5 she says to Mrs Grant:

“Well done, sister! I honour your esprit du corps. When I am a wife, I mean to be just as staunch myself; and I wish my friends in general would be so too. It would save me many a heartache.”

"Staunch" carries meanings of loyalty and constancy. So I think Mary has no intention of being unfaithful when she is married. This is because she thinks it would be stupid and would bring her heartache, rather than because it would be morally wrong. Nevertheless, I suggest that we have no reason think Mary guilty of s*xual immorality, either before marriage or after it.