thankyou Kerrycat. Like you said, it's a comfort sharing with people who understand.
We will never know what might have happened if we'd done things differently, but we do know what actually happened. Part of the problem with Bonnie was that her regular vet was on leave and the vet in the same practice she saw didn't do any risk assessment around a dental procedure which was what pushed her over the edge. She was old with kidney disease - the tooth needed to come out as it would have been painful for her, but if I'd been properly advised I would have opted for euthanasia rather than putting her through an ill-advised dental anaesthesia.
I have had some grief counselling with someone who is a former vet. Apparently pets with kidney problems can die quite quickly and often it's heart failure which is what happened with Bonnie and it sounds like your Nathaniel too.
I totally understand what you've done with the stuffed cat - I've had times where I've cuddled Bonnie's urn and told her how sorry I am. I'm doing whatever feels right. That included adopting another older rescue dog last week. I was getting dangerously depressed about it all - losing Bonnie, the vets (another vet took over the last few days, still not her regular one) and also missing having a dog and had to act. I did the same when my first dog died, and losing Bonnie has hit me even harder because of how she died. (Randy had cancer and I had him put to sleep with me there). I now have 3 spaces in my heart - 1 each for Bonnie and Randy and a new one for Ginny.
I will go back to the grief counsellor as I don't want to become bitter about what happened, rather move forward so I can remember Bonnie and the joy she brought me.
When I can face it I will go back to the vet practice. What happened wasn't full on negligence, but it fell short of good practice. (There was a mistake re blood tests plus as mentioned the lack of risk assessment, plus no proper advice on the last Monday).
Meantime I'm finding ways to create memorials for Bonnie. My latest is a Facebook page in her name!