Oh Lissie: I feel every emotion since I know them oh so well.
First off - I'd not talk to Susan again. It is a cheeseless tunnel. She does not, cannot believe she is wrong. No amount of effort on your part will ever make her see that. I realized this with maggot years ago. Anything she says can and will hurt you. Please let it go.
As far as her involvement with his family - it is almost inevitable. I never thought Chuck's family would accept maggot either. I don't know how they really feel deep inside, but I doubt they like or trust her. But he is their family and at some point they know he made choices that don't include me and they learn to live with it because he is blood.
Accept that she WILL be involved with your children. You will do best not to say much about her to them, to let them figure it out for themselves but also to let them make their own choices. Of course I would not want her at my daughter's birthday party. Especially at my home. But your daughter has a say in this and if she doesn't want her - she needs to let her dad know this. That speaks volumes. It is HER day. Even if Ash wanted maggot at her party, though (which would be a cold day in Hell), I would not agree if the party were at my home. I have my limits too.
Honey - he is not your friend. Friends don't lie and cheat to their friends. The best you can hope for is a businesslike co-parenting situation which is better than most of us have. I can't even talk to my ex but that works for me.
I know how difficult this all is but frankly, scenes like this with Javier and Susan just make it easier in the long run for you to move forward without him. He is a total JERK.
Wishing you a better day today and in the days to come. You are a very sweet girl and you deserve so much more than this garbage.