If you cannot find a more newt savvy vet, it may be worth while to return to the original exotics vet and request a cytology (if they did not do this initially). The cytology should give information like if it's a malignancy, infectious, etc. However, I appreciate that newts are small animals and it may be difficult to get a good sample for microscopic evaluation but I think at least calling the vet and asking if they would be willing to try would be beneficial.
As far as the newt being in pain - Florida Atlantic University's Institutional Animal Care and Use Committee (IACUC) has a paper that I found on line that states signs of mild to moderate pain/ distress in amphibians as "Decrease in avoidance behavior (e.g. when approached), abnormal behavior, reduced food/water intake, closed eyes, color changes" and signs of severe or chronic pain/ distress as "Movement aversion, lethargy, closed eyes, anorexia, color changes, flicking with extremities or biting of affected area, lameness and ataxia, accelerated breathing" I don't know how much this applies to caudates in particular, but if your newt maintains a good weight and continues about with its normal activity, I would venture to guess that it is at least reasonably comfortable.
I hope you can get a diagnosis! Please keep us updated as to your animals progress and I wish you the best of luck.
HJ