I want to say "Please give Cosmo more time" but he's your cat and only you can decide. I've had five cats that have gone through this to varying degrees, two very mild and the other three chronic, and they were all pole-axed by it at first. When I was both force feeding and hydrating mine, I was often in tears and beating myself up about what I was putting them through. In each case, though, after a few weeks they started to be their old selves and within a month or so it was an established routine and simply a small part of their very good lives.
You have to give Cosmo time to adjust to being handled a lot in a way that he's not going to enjoy. All you can do, I think, is enrich the rest of his time with you. Give him extra attention before the fluids, and give him extra lap time afterwards. Make sure that you both feel that the disease and treatment is just a small part of his life and not the only aspect of it. Focus on some fun things you want to do with him.
Regarding the fluids, they didn't tell you to give 350 at one time?! Sub-q fluids can put a strain on a cat's heart and the max I ever gave at one time was 200cc, to a 15 pound and youngish cat with no heart problems. I always found that by 150cc, the cat is getting super restless and it was only that Ted's ran in fast that I could hold him for 200cc. It's 350cc over the course of the next three days? If they want you to do this and then stop, I'm sure they will want to do a blood re-test on him by the middle of next week. At least, that's what my always did since it's the only sure way to know what is going on. At this state, and given Cosmo's poor initial reading, I wouldn't go on just my observations.





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