Stroke or brain tumor?

I have been doing a lot of searching on the internet over the weekend and am glad to have just found this forum. I hope that maybe I can get some experienced perspective on my Sheltie-mix's condition and what I feel was his untimely end. Max was a rescue dog so I'm not sure of his exact age, but he was roughly 12-13. Over the last two years or so he had begun to slow down a bit and act a little strangely. When I took him to the vet they said he just was getting old and had some mild arthritis, but now that I do more research I strongly believe he had canine cognitive dysfunction. He would occasionally splay out on our hardwood floors and we would have to pick him up, but once we steadied him he was fine. He would also occasionally jump into the wall when he would go up the stairs, but we also attributed this to his age. Four years ago he was diagnosed with kidney disease, but his blood work in early February showed his kidneys to be functioning fairly well. He had begun to exhibit incontinence more frequently, but it was not consistent. Max was wary around other people and for the most part only let me pick him and pet him, and I think he had been more stressed and confused as of late because we just had a baby in October and there were constantly family members and their more energetic dogs in his space. He still seemed to enjoy things most of the time, however, and he would eat and drink and occasionally enjoy being pet, although he seemed to sometimes get distracted and walk away.
On Thursday night my husband and I came home and found him splayed on the floor again next to a pile of his defecation. I thought at first that this was a normal episode of him slipping, and lately he had been defecating inside of the house even after I had just taken him outside, so I wasn't alarmed by it. When my husband went to try and pick him up, though, Max was snapping at him, so I had to do it. When I tried to steady him like always he wasn't able to stand at all and kept flailing, so I took him down the stairs to the backyard and steadied him. He stumbled around the yard in a drunken state, and he seemed very unlike himself. Both his back and front legs were stiff, his back legs more so than his front. He stumbled a few times so I finally went and picked him up and brought him back inside where he just collapsed on the floor. That night we kept him in our bedroom with us, and though he would stand occasionally and stumble in a couple of directions he would fall soon after. When he needed to pee he would stand up, but he ended up peeing on the carpet before I could get to him to take him outside. I gave him water throughout the night, holding it up to his head, but he was not interested in eating even a soft treat. The next morning I made him an appointment with the vet. When my husband carried him to the backyard he bent to defecate and did but ending up sitting down in it.
I was not at the vet's right when my husband got there with Max because I had to drop our baby off with my mom, but when I got to there the vet was performing a physical exam on Max. She showed me that when she bent his paws back (knuckling, I believe it's called) on all four legs he had no reaction whatsoever. She did a number of other physical tests that indicated to her that all four legs were somehow neurologically compromised, as well as his facial reflexes. She said she didn't think that he was in pain, but that he clearly had some sort of a neurological episode that was either a stroke or a brain tumor. When she finished the exam she didn't really say what our options were, I had to ask. She said blood work and x-rays wouldn't show much since the problem was neurological, and that an MRI or CT Scan could be done to determine the cause but because of his kidney disease those tests might harm him. She said given the extent of his lack of responses in all four legs and his age it was unlikely that he would recover his mobility, and if he did it would be months. She said we could just to take him or "it wouldn't be wrong to euthanize him." She said we had to consider his quality of life at this point. My husband and I cried and asked some more questions, but my husband thought it would be cruel for us to let him continue living this way. I agreed to it, and Max was euthanized while we held him on Friday morning.
Almost as soon as I got home on Friday, though, I immediately regretted this decision. I have been reading online all weekend about people whose dogs recovered from strokes, and I am angry that the vet or myself didn't think to at least wait the weekend out to see if Max would improve on his own. My husband is angry with me because he feels like we did the right thing, but I am devastated and feel like I killed my dog. Any similar experiences or feedback would be so much appreciated.

Angela

Thank you so much for your response. I considered having a post mortem done on Max to see if that would reveal what his problem was, but the vet said all they could was a superficial one that would not include the spinal cord or brain, which was most likely where the problem resided. It's just so difficult because last week up until his episode Thursday night that changed everything he was eating well and still looking out the front window like he enjoyed doing. I really do appreciate your time and input.