I reviewed this thread a couple times to try to figure out what the problem was.
Early on the only diagnoses was a curled gill. Healthy axolotls can have curled gills.
It appears that the filter was turned off in case it was the problem. This would be hard on water conditions.
The animal was fed generously and fed shrimp pellets. The pellets can swell and cause blockage. overfeeding can be a problem.
I advised balancing the tank, backing off on feed, and not feeding shrimp pellets.
The animal was mostly fridged. When you fridge an animal it is not in balanced water and often can not recover as quickly as in a roomy balanced tank at cool temperatures.
It looks to me like if the animal was kept in a balanced tank, with cool water, and fed lightly from the beginning it would have thrived. The big tank for a little axolotl would not even had to be balanced. It could have been set up the day the animal was received. It would have had adequate water quality and balanced quickly with partial water changes once a week.
It looks like the axolotl might have been compromised by to much tinkering. This is just an educated guess. Something else could have been going on.
May Thor swim peacefully under the Rainbow Bridge.