this has worked for me with frogs that wont eat but IVE NEVER TRIED THIS WITH NEWTS AND I DONT RECOMEND IT, but greatly diluted doses of liquid human baby vitamins, in a "bath" can stimulate appetite ..i think it was b vitamins that help , i cant remember fully what i did , but search around and check which vitamins are poisonous to caudates , and this may help, it is incredibly difficult to work out dosage though and you should maybe ask a herp vet about this, sorry your newt is so ill, it does sound like it was unhappy for a long time
if it goes terrestrial it will still take wax moth catterpillars , theyre hard to digest so pop thm with a pin first(the skin is very tough for a newt to break)
also if its a young newt, it may have parasites !
if it has been in a tank where it was bullied or badly neglected, some newts just give up
try and keep the water at 19 degrees, this is a good feeding temp for taricha, mine are in an unheated tank at uk temps, and that is as hot as it gets indoors at the hight of summer, it sounds as thpugh you should definately not let the enclosure cool for winter this year though