I would go solo if your husband is likely to make you on edge. Send your regrets for him, and RSVP for yourself.
You are clearly very cherished by the family and indeed part of their family and shared history.
Go and be part of this next happy chapter. And also what a lovely person you (and they) sound. Not many people step in the midst of grief, rather people disappear, how kind of you to be a substitute mother. Also not many people remember the generosity they were given, but the father sends his appreciation yearly.