Thank you so much for the support! I read all the comments when we finally got to bed at about 2 AM.
This is a short update. R survived the night and made big strides in the right direction. He is responsive, can stand, and can walk with only mild ataxia (stumbling, clumsiness). He's also acting more like his normal self, which means he is alertly looking for attention.
It feels like a miracle to me. When we last saw him, he barely knew we were present, was having seizures and all sorts of involuntary movements, and couldn't support his own weight. Both we and the vets feared he might lose his life yesterday.
The vets are returning to the original diagnosis of toxicity. The other things on the list were more scary but toxin ingestion is usually completely reversible if the dog survives.
He will be staying in the hospital. We don't know how long.
Thanks again. Blogville is amazing, and the POTP is magical.