I don’t believe that it is stated or implied that anything that happens in the Ter’angreal is true. Certainly it doesn’t make sense that Aginor would have been around to assault Nynaeve at some point in her past. It does seem to be the case that the three rings might have some element of past, present, and future, but that doesn’t seem to align as well with the things that Egwene sees. It’s also apparent that the things seen are associated with fears of the candidate. It may be that this is a representation of a past fear that Nynaeve had in order for her to face it and overcome it. So not necessarily something that happened but perhaps something that she was afraid of. It isn’t really ever made fully clear, but I don’t think it’s meant to be the case that Nynaeve was actually assaulted in the past.

The three oaths are generally grounded by the oathgiver’s grasp on reality. It is at least implied that the oaths can effectively be broken by someone able to manipulate her own view of reality to the point that she doesn’t believe she is violating the oaths. Verin is able to somewhat defeat the BA oaths by literally interpreting “hour of her death” when it might have been intended figuratively. Of course, the BA oaths is worded somewhat unnecessarily poetically. Or maybe it was a backdoor that some early reluctant BA member built in. That said, RJ treads very lightly in this area and I think that is for the best as a story needs to feel like it has rules and that breaking rules at least has serious consequences.