What a tough situation.
Normally, I don't tell BS to censor themselves at all. You let it out - all the anger, sadness, rage, everything.
But given that you have a LOT of years unaccounted for, I'd actually suggest trying to create as warm of a situation as possible. As in, make the questions or probing gentle. Be positive and supportive. I just don't think she's likely to come forth with anything if she feels she's going to lose you or you're going to be forever angry. I mean, she got away with it for 20+ years. If she feels you're going to respond negatively/leave her, she may roll the dice as she did with the others and hope they stay hidden for so many years. If she is lying, she may also feel confident that so much of the evidence is detroyed/non-electronic/lost if any additional As took place in the early years.
I also suggest you discreetly snoop like hell through her email, phone, etc. Things from 5,6,7 years ago could still be lurking in there.
I hate that advice, really, because you shouldn't have to make this easy for her. But I don't see how such a "successful" liar will stop lying (if she is) unless the conditions are right.
For what it is worth, double, all of us who have experienced an A go through this - what about all the other years? The other times? For many of us, we discover As we weren't even looking for.