floating tail up is air lock, the tank wasn't cycled and by cleaning everything you would have removed any bacteria forming, without the bacteria to consume/convert the ammonia the levels will go up, it isn't just ammonia that is tested for but ph/nitrites/nitrates if there are no nitrates then the tank wont have even started cycling. adding chemicals in a bottle may be a temporary solution it is not a permanent one.
although it is very nice have everything glowing, uv light will disinfect and destroy any bacteria.
your axolotl was to small for sand but I think that the water parameters caused him/her to resort to gulping at the surface and getting air trapped.
when you tub a axolotl that is floating you only use enough water so it can stand in the water until it passes the air.
axie poop changes dependant on what it has eaten.
fridging is a last resort when everything else has failed, never place directly back into the tank after fridging, they should always be acclimatised before moving to prevent shock.
if an axolotl is dead then rigor mortis will set in so it will become very stiff like a piece of rubber.