*But the issue isn't who she is. It doesn't matter who she is.
You don't trust your partner. Why are you focussing on her instead of
dealing with the glaring issue in your relationship? What use will it do
to find out exactly who she is? What difference will it make*
Not to be rude but we don't live in a vacuum, we make decisions and choices by the actions of others, where there was once certainty, changes can occur, op is trying to acertain wheter her assumptions are based in reality or whether her mind is working overtime.
No relationship on earth, whether it be romantic, work, family or whatever else is founded just on the basic fact of do you trust, do you not trust, it over simplifies things too much.
I don't trust you, therefore get rid, we may feel like this one day but another day our fears maybe dispelled.
Op, trust your gut, it sounds as though he may be lying about this woman, to what extent you will have to find out.
I hope it's not bad news.