"The OW here was too old to have kids and said she didn't like them, think she just liked sex, wherever and whenever she could get it and not just with my poor pathetic shit of a partner either."
OP, how do you know these things? Who is the source of this information? I suspect this is what your husband has fed you. So, not content with stringing her along for TWELVE YEARS, he destroys her reputation too?
Try, please, to look at this situation without letting your (entirely justified) emotions get in the way. TWELVE YEARS. Four thousand, three hundred and eighty three days. And he has lied to you on each and every one of those days.
He is not a numpty, he is not pathetic; he is a very, very good liar. An extremely successful liar. As such, anything he tells you needs to be verified by independent sources before you can believe it. Indeed, you would do well to be suspicious of anything he tells you, because to deceive you for so long, so successfully, he must be very good at misdirection. Misdirection is making you look in the wrong place, so that you don't see what he doesn't want you to see. He will say things to you to achieve this misdirection. It seems to me he has told you things about this woman so that your focus will be on her, sneering at her, accusing her - because while you are doing that, you are not looking where you SHOULD be looking which is where he doesn't want you to look. You are not looking at HIM and at his behaviour. And you really, really need to look hard and to look close at HIM.
Do you really think that any relationship, even one that is on-off (and remember, you don't know that it was, you only have his word for that and his word is a lie) that has been going for twelve years does not involve emotions? What is more likely - that she is a bunny boiler (and that's an awfully long time to come to the boil, don't you think?) or that she has been lied to and strung along? Look at it cold and hard - you KNOW which is the more likely!
Oh, and the shagathon? He is the one who had a shagathon! He didn't fall for 'she wanted to see the view'. That is just another lie, to add to the many others. Did you really believe it? Maybe you did, after all he is a very successful liar. Or are you still smarting at having been lied to for so long that it's hard to admit that he is STILL lying to you?
OP, you're entitled to be angry with her; but please, do not let that anger blind you to the real culprit here - HIM. He is the one who you should direct your full ire towards. He is the one who made vows to you. He is the one who had lied to you daily for so very very long. He is the one who took another woman into your bed. And he is the one who is STILL lying to you.