OP, you are in such an awful situation, but you cannot contact her to have an honest chat. You won't get the answers you want. Based on all the messages you've described, and your husband's attitude, nothing will be admitted to. At work it's either more of the same between them, ie flirting, or they've taken it further.
Make a decision for yourself and your children about your marriage and home life because you already know your husband's decision. Please plan your own future now and put those plans in motion sooner rather than later. I wouldn't have further discussion with DH. I'd be civil, but I'd be obtaining legal advice for separation, full custody, retaining the home, and financial support now.
Having said all that, if it were me, I'd also do my best to fucking humiliate him, and her, after kicking him out and servi g papers, iie an announcement on FB:
I just want to advise my family and friends that I have kicked DH out and served him with divorce papers due to his intimate relationship with his colleague X. To prevent any misunderstanding I want everyone to hear it directly from me. I'd also mull over confronting her angrily in public.
But, those are my musings...
OP, you deserve a devoted and loving partner. Life is too short to suffer when you can move on to so much better.