I'm completely new to axolotls, but I have kept other amphibs and fish my whole life, so hopefully this isn't all way off track...
0.6ppm ammonia is BAD and deadly. What test kit are you using? A reading between 0 and 0.6ppm is hugely inaccurate, is there another test you can use?
How often are you doing water changes, and what are your water parameters normally? (pH, Ammonia, Nitrite, Nitrate...) You've only had these guys 5 months (or less- I'm just going by Spunky's age)- was the tank cycled before you added them, or was it a fresh setup? If the system was already established and you're doing regular maintenance your ammonia levels should easily stay at 0. If it wasn't cycled, and you're doing regular water changes, cycling the tank takes longer and having the axis constantly exposed to fluctuating levels of toxins would slowly weaken the animal's immune system... feel free to correct me if I'm wrong, as I'm mainly experienced with fish. Either way, ammonia should be 0. The fact that you have traces of both ammonia and nitrite in your system is a big red flag to me- can you tell me EXACTLY what your tank maintenance schedule is, how much water you change, etc.?
The other problem I see is temperature... what is the temperature normally? You mentioned 25 as a high, but didn't say how much higher than normal that was. All the research I've done says 60-68 F or 15-20 degrees celsius as the acceptable range. Ammonia toxicity increases as the temperature and pH increase. Oxygen levels are also lower in warmer water. I don't know your pH, but the warm water/low oxygen combined with the poor water quality could be what's causing, or at least contributing to Spunky's condition.
... Just reading about bloat in newts and salamanders now, and everything I've read says that heat stress and icky water can cause kidney failure, which shows itself as bloat. Bloating could also be a symptom of some kind of systematic infection... If Jynx isn't showing any of the symptoms, it seems like kidney damage would be the culprit rather than a abcterial infection. Here are two of the sites I was reading :
http://www.caudata.org/cc/articles/bloatEDK.shtml
http://www.livingunderworld.org/amphibianArticles/article0001.htm
Some others said treating for secondary infections with an antibiotic might help, if the tissue damage wasn't too advanced, but overall the prognosis wasn't encouraging
Putting them in the fridge also seems to help them recover on their own- does anyone know exactly what it is about being cooled that helps so much?
Has Jynx shown any of the same symptoms? If it were me, I'd be really aggressive with tank maintenance and keeping the water temperature down. Even if it is too late for Spunky, it could keep the same from happening to Jynx!
I really hope he can recover from this, it's so hard seeing them suffer!