Change your vet. And I would think it was down to blat or gastric torsion. The vet should have been able to answer this for you though. I hate an unsupportive vet.
I'm not sure what a doxie is but FYI many dogs are suceptible to bloat if a large breed like a bully, so vomiting should always be taken seriously, unless an obvious reason for it such as scoffing too quick or eating faeces....
I am sorry about the loss of your dog. My first thought is have you sprayed your floor boards with any insecticide? The other thought is the food , and then one more thought is that he might have been ill for a long time and it just happened. My dog with a low thyroid drinks water a lot. This has to be tested with a thyroid blood test, and it is easy to over look as when the dogs are getting older we asume they are just getting slower. When the thyroid is low it will start to cause other problems with the body. The hair will have issues, and the head shaking might have been mini seizure ( hey, I am just guessing on all of this) Your vet does not seem like one I would trust as he had accused you of harming your pet, he was not looking to help you any more. You did nothing wrong, but I would look at finding a good vet.
Oh boy...I am so sorry to hear about your dog.
He was drinking excessive water...you had him checked out, and he was found to be OK. He had the sores on his back...you had him checked out again. I honestly cannot see you have done very much wrong here, and you did try for him.
The only thing is, it is strange for a dog to be drinking excessive amounts of water, and when tested, for there to be nothing at all wrong....unless it is very hot weather, or the dog has been exercising hard. So it makes me wonder if your vet missed something.
What happened to him may have had nothing to do with eating the food which spilt on Sunday. The only thing is if he kept vomiting during those night hours in his kennel, I guess it is possible it could have led to severe dehydration. Otherwise I can't tell what might have caused him to die and so quickly. You probably couldn't have known, just thought his tummy would settle in the night, and he would sleep it off. They often do. Even if there had been a big bowl of water in his kennel it may not have helped, as sometimes dogs can enter a vomiting/drinking lots/vomiting cycle, which can lead to dehyration anyway.
I am so sorry this happened though. Take care now of your other dog, as he will be missing his buddy and brother too.