I agree w/ you 100% that it would be fruitless to go fwd w/ MC if she's actively engaged in an affair.
But, I would also make it clear to her that while you do not condone the affair, that you own up to your shortcomings in the marriage, and that you are working on becoming a better partner.
I just don't want you to go in there guns blazing like it was all her fault. The affair is her fault. But the failure of the marriage -- that's on both of you.
My two cents.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final