I agree with Sandi of course-- do not confront about the flirting, she can do what she wants.... people make their own choices and you can make your own boundaries. but if you confront it... sounds like an ultimatum.
Also, I could tweak your response to the sex ambush...how about "I understand..." and thats it. No follow-up, no checking on the future... etc.
I am not an expert, just sharing a couple of ideas ...
H (me) 52, W 42 M 15 D14, S12 PA June and Sept 2016 Found out Sept, confronted Oct NC with OM since Oct, remorseful Dating since Nov