The person that is planning to get married to your stalker ex is looking for anyone that will validate her position. Of course you qualify for the job; you helped bail her out the last few times from what I read. So she figures that if she leans on you enough, you will cave in and give her support again. For the guy will getting back at you again for not taking care of him all these years will be just a sweet bonus for him. It sounds like you know what you are going to need to do. It is okay to ignore these two. You can't fix everyone. You clearly have a big heart, and want to help however you can, but there are some people that the best help you can give them is none. As long as you continue to help them out, they will be victims.
It is like parents that cling to their children long past adulthood. These people are often not allowed to ever be adults. The parents will coddle and continue to care for them as though they are small children. So that by the time they should be able to provide for themselves and care for children of their own, that these Adult/Infants, are now only capable of remaining home with their parents. The Adult/Infants will throw tantrums and be incapable of any means caring for themselves, and yet will want their immediate desires catered to. They will rely on anyone around them to care for those needs, and will look for people that will fall for their behaviors, so that they can have instant gratification of their wants. They in effect become retarded. So as long as they can manipulate the adults around them they will. As long as someone will feel sorry for them, and bail them out again, there is no reason for them to grow up get a job and support themselves, and their offspring. These people are retarded by their own behavior. It doesn't sound as though they are people of limited intelligence, or physically handicapped. Yet they are living as though they are both.
Having had a daughter that was both of limited intelligence and physically handicapped, it burns me to watch apparently normal intelligent adults acting like they need to use up others time and energy. There are enough people that really need help, that it would be better to spend your time with someone that really needs, and wants the help and love you have to offer. I can tell that these two will suck you dry, use up your resources, and never appreciate anything you do. She is not a friend she is a vampire, that will suck you dry, and then she will still be p#@sed off that you didn't do more. These people will say, and do all the right words, as long as you are doing what they want you to do. As soon as you stop, they will turn on you like a pack of dogs, on a chicken.
I would cut your losses now, it won't change the outcome. The trick for you will be filling up the emotional hole which her loss will make for you. You have spent quite a bit of time caring for her and her daughter so there will be a gap in your life. I think the hardest part about letting go will be the not knowing how the story ends. But you can always hope that in a few years, she will come back a be able to say that not helping her out was the best thing that could of ever happened to her.