I'm really sorry you are having to deal with all that garbage. It does sound like G isn't really playing ethically in his relationships. Neither is U for that matter. One lesson I have learned from poly is that you NEVER "wait for someone to come around" to either monogamy or polyamory. That is wishful thinking and that ends up in people getting hurt. It is also incredibly presumptuous. Every time I have met someone who indicates that they think they could do "better" for me or shows signs that they think polyamory is just a phase or that I haven't found the right person yet, I send them packing. I'm sorry that G hasn't learned the same lesson and that you are suffering the fallout from the drama of it.
It also seems like G isn't really being honest with you. Did he not think it was important to let you know when your metamour, his wife of thirty years, was suffering from health problems? He sounds like he is doing a lot of wishful thinking and pretending that polyamory works for him, while not actually doing any of the work that makes it a healthy relationship structure for him or his partners.
I don't think it was necessarily wrong of you to voice your concerns about U. You didn't forbid him from seeing her, but you did let him know that you saw potential danger in the relationship and you were right. I'm also sorry that his family did not let you go to H's funeral and that they are blaming you for her death. Please do not blame yourself and don't let them make you feel as if it is your fault. You cannot be responsible for another person choosing to make healthy decisions with their lives.