Do you still get along with your ex? Do you think it will be awkward to be around him if you go? What if he brings a girl as his date, will you be upset? I say if you being there won't cause any problems for anyone else, then go for it. But if you think even a little bit that feelings about your ex may get in the way or even pop up, I say no. This is her party and you don't want to do anything to ruin it or distract it. And you may consider your ex boyfriend too. Will he get mad at his mom for inviting you? You don't want to cause any problems between them, even tho she invited you. So if you think any ounce of problems may arise, just take her to lunch one day just you two, and celebrate.
Not unless you want to be with him in the future. She has an ulterior motive for inviting you, like getting you back together. Unless: you are an adult (similar stages of life possibly) and your boyfriend does not live with her.
go for it unless it bothers you being around him because this would not help you. if you are not affected in a bad way by the slit up and you care for her and like the friendship with the mother go for it.
If youre friends with her then sure. Lots of ex's manage to tolerate each other and even be friends and the ex has nothing to do with your friendship with the mother. You do need to watch out because mom may be hoping the two of you will get back together.
No, the remaining friends with ex's relations isn't always the best. That said, of course you can remain friends, in a casual way, FB, instagram, whatever, but attending family functions? No. Sounds like you still like this guy? Hoping to get back? Send her a card, mail a gift. He probably isn't comfortable with you hanging out with his mom. What if he brings a date? Best to let it go for now, unless he gives the ok.