In your opinion, is there ever an appropriate time to ask her about the A? I'm prepared for whatever the answer is...
Ask about the A as in "I need to talk to you .. is the A still active?"
In my opinion this should only be discussed when the WAS comes to the point that he/she is ready to commit to the M, no elephant in the room, no going through the motions. Coming to the LBS and saying "I want to work on the M and I will do what it takes to fix this" It is only at this point can you make sure the A is over and there is a NC in place with the OP....With hopes that they recognize the damage they did and own up to the A and prove to you they want you and the M and severe all ties with OP and the A.
Truth is ... there is no point in asking about the A as you are in that limbo spot right? 3 responses you would get at this point... 1.)She either tells you "yes I am still having an affair" ... which crushes you as you think you are so close to piecing. 2.)She says its over and has been ... but you have not and will not have any way of knowing as she isn't being open and transparent with you. 3.) She says its over but lies as its deep underground ... again .. no better off now than you have been.
My concern in your sitch ... you are allowing some cake ... I get your angle but I fear if you DB this half ass you are going to either live in denial about the A, have no closure ... if you do not address this ... what consequences have there been? Even if the A is over... is there a chance for OP2?
I get you own your faults in this .. but strike that 75% your fault thing .. you can only take on 1/2 .. thats 50%, the A was her 100% choice ... the issues in your M ... you get yours .. she takes hers.