Totally agree KML. I learned that with my STBXH's daughter's mom. My STBXH made her out to be a crazy person. She definitely has that side to her but she is not nearly as evil as he made her out to be. Certainly, he strung her along for as long as he could before she found out about the double life he was leading back then. It is a pattern of his and she was angry about it for a long time. I totally understand why. Anyway...in the beginning, I was too involved with the drama and it didn't help things. Eventually, I took a step back and treated her with compassion and empathy even though I still had some disparaging thoughts about her. I did it for my stepdaughter who didn't need that much animosity between the parental figures in her life. SD's mom now wants to be my friend but I'm lukewarm on that idea. Luckily we don't live in the same community.
Re: M's ex... all I can say, as the mother of two children who are spending 50% of their time with my STBXH and his affair, it takes a really strong person to go to an event like that and not be affected emotionally by it. I have not yet had to go to anything with the two of them playing happy family but I'd like to think that I could manage it even though I'm sure it would anger me on some level. But I'm a strong person and I am well aware that there are many people out there who are not and would act the same way his ex did.
Also...again, a word of caution... there are ALWAYS two sides to every story and you have to take into consideration who is telling it. I'm sure my STBXH has told his affair all kinds of half-truths and untruths about me. Why would he tell her anything that paints him in a bad light?? So...I'm sure if you talked to M's ex, she would have a very different account of their history. Where does the truth lie? In my experience...usually somewhere in between. My advice to you, as someone who is just entering the picture and is not the kids' mother...be supportive and kind and all of those good things that you are but steer clear of getting involved with the drama. That's M's drama to deal with - he had a hand in making it so he needs to manage it.
BTW...if she truly is evil and a horrible mother, perhaps you should be making a report to child protective services.