Cinders ((((((((((hugs))))))). I'm so sorry all this is happening to you.
About H and what your S4 said to him. My kids are all teenagers (well nearly) and they too have said similar things to my H (although obviously in more grown up language ). It's usually my D12 who is most prolific with this. When she does my H blames her sister! You see my D17 used to be our 'problem' child and certainly can be very quick with her tongue when she want to be BUT since H left she has turned her life around and become such a rock for me. I hate that my H puts all of this blame on her. However the point is that it doesn't matter who they blame, we know the truth. IMO they also know the truth but they just don't want to admit hence the deflecting of the blame onto someone else. That's also why he wasn't happy with your TM he knows it was wrong but doens't want to admit it.
WRT to your dog can I just say one thing. I don't know if you have prepared your kids for what you are about to do but if not please do. When I was about 10 I went out to play one evening. Our dog, who had been my constant companion and protector since the day I was born was old and his back legs had gone. He was not well. My dad took him to the vets to have him put to sleep whilst we were all out playing. It was a summer's evening so it was light nights. When I came home Pete(the dog) was gone and I had not been given the chance to say goodbye. I love my father dearly but after all these years I still find it difficult to forgive him for this. I know he did it b/c he found it so hard himself, the dog would do anything for him and they too were constant companions BUT even children need to be able to say goodbye.
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15