My knee-jerk was to put her back in the fridge, since she was doing well there. However, it could just be the change in temperature, so after thinking about it a while, I think it would be best leave her there a while and see if she improves.
I don't think the water is too deep.
Is she surfacing (or trying to) for air a lot? What color are her gills? How often is she flicking them?
What temperature is the water at? I know it is summer over there.
Was she lethargic after she warmed up but before you put her in the tank?
Observe and report as any details about her appearance and behavior as you can.
So, here is a synopsis of what started this off, to get everything in one place.
0. Hikaru is 7-8 years old and was given to them a few months ago.
1. 3 weeks ago, Hikaru was subjected to a 100% water change with warm, undechlorinated water (not by the OP).
2. She stopped eating and and spent most of her time floating and limp.
3. After 2 weeks of this, OP was going to have her euthanized, but found this site and decided to give her another chance.
4. Hikaru had been in the fridge for that past week, where she has been acting much better, but still not eating
5. OP spent the time learning about the cycle and more about axolotls and has procured such necessities as dechlorinator and a water test kit.
6. Yesterday, Hikaru was slowly warmed up and returned to the thoughly-cleaned tank.
7. She went back to being limp, but she is not floating.
I am hoping some experts on axolotl health can chime in with advice.
What I am wondering, could her ordeal have somehow impacted her ability to get sufficient oxygen from the water? That would explain her activity in the fridge, and her limpness now, since cold water holds more dissolved oxygen than warm water.
Here are my suggestions.
1. Add an airstone if you can to better oxygenate the water.
2. Cool the water with ice bottles (This isn't something I usually recommend and isn't a good long term solution, but might help Hikaru in the here and now.)
3. Have a fan blowing across the top of the water. (This will cool it and help with oxygenation.
4. Watch her behavior for improvement.
Get well soon, Hikaru!