I have an older Amana SRDE25TW which doesn?t freeze/cool properly

BACKGROUND: I have an older Amana 25 cf side-by-side refrigerator/freezer (SRDE25TW) which although a 1996 model was put into service in early 2000. It was top-of-the-line in its day, with even a fancy electronic key pad in front. It worked flawlessly for years. Last year it started to complain frequently of "high temperature" with a warning beep and a red light on the panel. An alarm reset and setting both the refrigerator and freezer temperatures to the maximum would solve the problem for a few days but soon it would do it again. However, the problem got worse as it kept happening very frequently. It peaked once when apparently it got so hot that the compressor refused to work (or vice-versa?) A $75.00 service call taught me that a cool wet towel around the compressor and a thorough scrubbing and vacuuming of the bottom outside coils and nether areas underneath the refrigerator would get the compressor and the cooling working again. I also learned from the technician that when the fridge is left on the highest temperature settings, the interior coils in the freezer tend to frost over and cause all kinds of temperature problems, including the ice maker line to freeze and stop working. I learned to uncover those coils (at the rear of the freezer), disconnect the thermister and defrost the coils with a hair dryer. I also learned the hard way that the coil cover panel had to be secured and the thermister reconnected properly for the thermister to get a correct temperature reading and work as it should. For over a year now I have been nursing my old fridge along this way. PROBLEM: However, a week ago, with only a couple of high temperature warnings, it stopped making ice, the freezer side defrosted completely and the refrigerator side stopped not cooling fully. As usual, I cleaned the dirty bottom coils but, when I checked the freezer coils I found they were not frosted over!? When I removed the thermister (Amana - Whirlpool Part No. 12673301 or Maytag Part No. C8983701) I also found out it that it was missing its round metal cover (it's nowhere to be found) and that of the two thin filaments s inside, one is missing what appears to be a miniscule red mercury tip. When I reconnected the thermister the electronics control panel went crazy beeping and flashing and eventually most of the lights and the temperature indicator bar just turned off. Now neither the freezer nor refrigerator sides work at all.
QUESTION: Could it be that the faulty thermister was the source of the entire problem since the beginning? Could it be that the small mercury tip fell off inside it a year ago but went undetected because it was behind the small metal plate? Could it be that by missing the small metal plate/sensor and the mercury tip the thermister kept misreading temperatures and eventually became useless? Also, I also always suspected that the system might be low on refrigerant. Any thoughts on that? Could it be something else altogether?
Any suggestions will help, before I decide to a) purchase a new thermister ($30 to $50 online) and/or b) call a technician for another expensive (and possibly fruitless) lesson or c) get a new refrigerator (a comparable one now runs between $1,300 to 1,600….
Any thoughts, solutions, comments or questions will help. THANK YOU!