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TL;DR
Fortune is stable, and improved from this morning, possibly from the subQ fluids from the vet visit. His neuropathy has improved with treatment over just 24 hours and prognosis is good that he will recover; it suspected to be a complication of his cancer because of the rapid onset and positive response to the treatment vs diabetes or low potassium vs autoimmune. Right now we expect to see improvement over the weekend, and when nausea/pain/lethargy are improved (hopefully they are due to chemo) we will increase dose. He gets very tired right now and has to rest often between activities.
Internist feels his CKD is "stable" and is "fine" with his diabetes treatment.
Fortune ate half his breakfast (1 3oz can) with the gabapentin and the potassium when we got home - yay! He was cuddly and much more himself this afternoon, and we are hoping he is getting what he needs to heal. We will know in the next few weeks, but for now we are grateful to keep lovin on him.
Longer version
We got home a few hours ago. Fortune was NOT critical - there was some awful miscommunication on the part of the vet staff. They told ME that Fortune was in critical condition pass on the symptoms I reported, and that he needed to come through the ER, not his regular appointment. However, his status has not changed except that he was a bit more lethargic and looked somewhat pained. Fortune didn't have enough time to eat before we had to leave for the vet - last night he needed almost 1.5 hours for the meds to kick in. He is clearly more nauseated - hopefully it's from the chemo and will resolve soon.
2.5 hours after our original appointment, the internist called us. Fortune "looks ok." We discussed repeating b/w to see where the potassium is, but he just had labs done yesterday and only had one serving of the potassium so we held off. We are increasing his ondansetron to 3x/day to see if it helps (how do folks work out the timing for 3x/day meds?).
I inquired about subq fluids, however she doesn't know if it will help him, because he didn't "feel dry" to her. She agreed to give him fluids today and if it helps to revisit.
I pushed on his prognosis - and was told that the HSA is driving prognosis not his CKD or diabetes. She can't "stop his kidney from failing," but the CKD is "stable" and she is "fine" with how we are managing the diabetes. She also deferred to neurology on his weakness when I asked about how he looks. Because of his rapid progression, she felt it presented as neurological. No mention of phos binders or low phos food, even though he has high phosporous levels.

I don't really trust that she is giving him the best care, but right now I am not going to move Fortune to a new doctor because he is getting great care by the other doctors.
The neuro visit went well - I better understand what is going on now. Fortune's peripheral neuropathy MAY be caused by diabetes, but it is unlikely because of how quickly he progressed and how well he responded to the mestinon. The neuro was "very impressed" by how he responded on two small doses over 24 hours - yay! She believes that it is secondary to his cancer, and it affecting his nerves. She did not give us a name for it, but I did ask where I can look to become smarter and better help Fortune and I hope to get some more information in the coming days from her and the medical notes.
His prognosis with the treatment is good, and she believes that because of his good response, he will regain the ability to walk, jump, and move more easily. Right now, he needs to rest between activities because he is weaker. She basically described spoon theory if you're familiar with chronic illness (I'm a spoonie). We are going to wait to increase his dose until he has less nausea and feels better, but hopefully over the weekend we will see continued improvement.
So after the sage of the past three days, I feel like we really truly need a vacation. We went through all of 2020 without going away (or even out to eat), and I could deal. But the total and compete exhaustion is getting to me. I hope we can have some stability now that we have some answers.