I think I would have gone along with the Christmas plans with the cousin staying , after all , Christmas is for families and the cousin is family , you may have found that you had a lovely time rather than stay behind on your own, more the merrier and all that . , after all , they were happy to include you with their celebrations . As for the puppy staying with you , that were easy to reply in saying that as you’ve got the cat that it wouldn’t work , so, for that reason you’ll have to decline ?
Regarding the money , if they can’t afford to pay you back in one go , can’t you all come to some agreement that they pay you back in instalments.?
The ‘ wife ‘ situation is something that’s not uncommon, you hear about it all the time , thing is some men will hang on to their wife's every word , just look at the H&M situation! In that situation, whatever you say you’ll never win over the wife , so, best to take a back seat and let them get on with it . Best to make a life of your own with your own friends , get involved in groups or even volunteering , then , when an invitation comes around to spend time with your son , you’ll be in a much better place to accept and enjoy .
Must say though , that I do find them both very thoughtless & selfish in what they did with lack of wedding and graduation invitation. I often find that some people are happy in their own little bubble and don’t care who they tread on or hurt . Maybe a heart to heart with your son be a good idea , that’s only if you already have a good relationship with him and you think that would work , otherwise, do as your ex husband suggests and say nothing.