Thank you for all the comments and kind wishes. This whole thing has been really sudden and overwhelming and there weren't many people at the time that I could reach out to for support, so it really means a lot to me.
Unfortunately, Murphy had gotten much worse today, and it was clear he wouldn't have recovered from surgery, so we chose to put him down. He had been getting weaker every day, but before today he had still always at least had spurts of energy to do his favorite things, like chasing squirrels and playing with his toys. But today I still couldn't get him to eat anything at all, and he was having a lot of trouble climbing the steps on our deck and getting in and out of the car.
It was so hard to see him like that. I felt like I was watching him die. I definitely feel like we made the right decision. The vet told us that if we had caught the defect sooner there was still no guarantee that we would have been able to save him. It was a liver shunt, which reroutes blood around the liver so the liver becomes very small and the body fills with toxins. It's very serious, but, like in Murphy's case, it's possible for a puppy to live a fairly long time without showing any symptoms at all.
I'm glad we were able to give him a good life and a loving home for the short time he was with us. I will never regret picking him out. He loved everyone he met and he made us so happy to be his family. He was a really special dog.
I don't know if there's a heaven or an afterlife, but it makes me happy to imagine that he's in a wonderful place now where he's playing with other dogs and doing all of his favorite things. Rest in peace, Murphy.