Couple of thoughts on this.
(1)
changing of the age of consent - 13 years old raised to 16 yrs in England and 18 years in Ireland.
My main female character gets kidnapped and sold to the highest bidder in a brothel.
Raising the age of consent was a very good thing. Is it related to what happens to your FMC? That is -- would your FMC have been protected by the change of law?
Was the FMC herself underage when kidnapped? I hope not.
(2) In the FMC's situation, that particular age-of-consent law wasn't necessary for legal action. Kidnap and rape of a well-born virgin -- whether she's 17 or 37 -- would be quite sufficient to get everybody responsible hanged
... including the MMC.
You have probably tidied up the loose ends as to why the Crown didn't prosecute him and why the FMC's father doesn't demand it. Why her brother didn't round up a gang of friends and horsewhip him.
(3)
Fast forward several years the main male character, who is also upper class, has realised the seriousness of his actions and decides he needs to marry her as it is the only way to restore her standing in society.
Here I wonder why the FMC's family didn't demand the MMC marry her, back in the day. Failing that, why they didn't, in several years, marry the FMC off to somebody else?
Could it be the FMC say, "I will not marry."
As it stands, she's suddenly willing to marry the MMC, (kidnapper, jailer, fumbling rapist, buyer of unwilling virgins.)
This after several years, when she and her family haven't found
one appropriate ambitious young man of the lower classes who wants an entre into Society, one yeoman farmer making the jump to country gentleman, one curate who believes in forgiveness and wants a country living, one scholar at Oxford who will marry to have support while he goes on with his work.
But if you make her adamantly against marrying, there has to be some reason she changes her mind. It'll probably have to go back to the period of the kidnapping.
(4)
the more I find out about Stockholm syndrome the more I do not want it to happen. I will not permit my main female character to fall in love with the male character while kidnapped but the marriage is a different kettle of fish.
I don't see a problem with the FMC
not falling in love with the man who holds her captive. Stockholm Syndrome is not particularly common.
You have infinite power in the story.
You get to decide it doesn't happen and write it that way.
No one's going to expect it.