I seem to recall the WN only had the original Weiss grope her, but the this version has the original Weiss attempt to rape her. Honestly if she's immediately in love with him like in the WN; that's just poor writing... her dancing with him after just a single apology is already pretty damn unbelievable.
I mean, it was similar with his maid; he was whipping her, stopped and she immediately had no issues being loyal to him. None of his servants bore any grudge towards him either despite how he treated them. I guess that in the servants' case, they might have been taking care of him since he was a small child, so they might be fond of him and more willing to overlook the time when he acted rotten.
As for Cynthia, well, if she followed him to an empty room, I can't imagine she wasn't somewhat interested in him to start with. He was someone of high status who had shown interest in her and that's always flattering. I don't think she's forgiven him with just this, so maybe she's just probing to see if he's really turned a new leaf. In the previous chapter, she had read the article about him being involved in the arrest of criminals and did appear intrigue so she might believe that this apology confirms that he's changed. At the very least, she doesn't have any trauma from what he attempted to do to her, since she's confronting him fearlessly.
We'll just have to see what she intends with the request to dance, maybe she's just doing it because the dance part has started and he's a partner that's available, maybe it's because he had done a public apology, so she might to show through a dance that she has accepted the apology and not embarrass a person of high status, maybe she wants to see how he actually behaves while dancing to confirm that he won't attempt something like that again, maybe she's going to repeatedly stomp his feet during the dance as payback (it appears that in the original story, she was quite vindictive and apparently involved in his death) and call it even as long as he just lets it slide and then reset their relationship to neutral.
It might be something dumb for comedy's sake. For instance, from his side he might have difficulty believing that she's forgiven him and his actions might show that, which might trigger her to act increasingly sweet towards him in order to show that she has indeed forgiven him.