Oh Kaykay, life just is not fair. I really do feel your pain.
I don't know that you will ever get answers to all your questions.
I think that you just have to treasure the memories. Think of all the good times you had together and of all the love you shared. Know that he fought hard. You all did your very best to get him a good quality of life and to keep him with you. it just wasn't to be.
I pray you find the strength to get through these very difficult weeks and months.
Tammy