Vampires, werewolves, zombies with guns, a castle that behaves like a TARDIS as far as internal consistency goes, summoning feng shui magic animals, cream pies that deal holy damage, motorcycle-riding skeletons--totally legit.
A lady (who has supernatural powers already, a good start in being 'different') shirking a social norm--strains belief.
Not that I'm not for historical accuracy in things where it really matters, like I don't want to be watching Little House on the Prairie and then Elvis shows up on a jet-ski but...really now. If you wanna be that way, do it and boldly, but don't blame it on historical accuracy in a game where Richter Belmont the vaguely European vampire hunter is dressed as an anime martial artist. If the non-fictional Ching Shih can get the entirety of the Chinese government by the shorthairs, after taking control of and vastly improving a pirate fleet, having started out /as a prostitute/, I feel like a lady can be a Belmont with people still being on her side over Dracula's, in a work of not even remotely realistic fiction.
As to her name having to stay 'Belmont'...can't she just fight vampires before marriage? I guess I'm not seeing the issue.
They wanted Sonia and Victor to co-star, right? Get rid of more time travel BS, make Victor the reluctant good-for-nothing brother whose duty Sonia assumes, then he makes a turn-around after being completely shown up by his sister, when things are looking dire. I'd play it.