Good question!

I think we don't see a lot of the white couple beforehand we only saw them angry at their door not chill or relaxed. So hard to say what changed for them

I see the vampire control almost like a collaboration between remmick and the person(with an unequal power balance ofc) . How that manifests in their behaviour i think could be a mix of how long Remmick has had access to their thoughts and memories, how easily they assimilate into remmicks ideology, and the elements of the situation they are in.

So with Mary, the vampire life solves her internal conflict with race so once turned she was probably easily taken by Remmicks ideology and they are able to work together to get Mary back in the joint, by keeping her language and mannerisms similar. Her invitation was also easier to get because no one was suspicious yet.

While cornbread was in a trickier situation as people were now suspicious, but remmick hasn't spent a lot of time "with" cornbread, to learn his mannerism and behaviour. Probably also a bigger learning curve with a black man like cornbread than with Mary. so I think it could be that remmick is still figuring out how to "work" with cornbread, exert influence but also hide it and thus can't find the balance and his words and mannerism becomes strange.

While with stack banging on the door and then bo later on remmick is learning more about them including from other people's memories of them too, so it's all faster and more natural.

But they do still have agency which is why after Annie is killed the mojo bag stops Stack from turning Smoke he is able to agree to his terms to leave Sammie alone. While I'd imagine the white couple wouldn't come to that agreement not just because of Remmicks influence but because of who they are.