I have watched a few You Tube videos pertaining to this issue, not convinced what the problem is. If you search water accumulating inside, most say stopped up drain. However, that wasn't the only problem in this case as there was over accumulation of ice in freezer part.

After leaving it sit a few hours, what had been a dry defrost tray next to compressor, was full of water, so I think that rules out a stopped drain? If it was stopped up, it would have leaked/drained other places.

Anyway, the defrost thermostat was bulging, and even though the test with multimeter showed it had continuity, (we did put it in freezer to get cold before we tested it), I ordered a new one as video said to replace if bulging. They are not expensive so worth a try. Heater/defrost tested ok, so unless it is the defrost timer, IDK what the problem is.

I've had one a few years ago leak water into the fridge compartment and also have ice in the freezer. When I pulled the plug to "reset" (as I often do just to check as sometimes that's all it needs and/or some cleaning of the coils, etc). I found a pea in the drain tube.

Yes Jonny, we were fortunate enough once to have a perfectly good unit given to us that we used for years in a rental. All that was wrong with it, it was stopped up in the drain tube, and you could get to the tube and we vacuumed it out. Green algae plug had stopped it up.

I have had the timers go bad. Easy enough to run one manually with a screwdriver and see if it does what it should. But it might not turn the defrost off so watch it.

Chances are you do not have a freezer t-stat, the freezer takes the fridge compartmnt overflow even on many side by side units, the air flaps often get clogged with plastic bag tearoffs from sliding things under the racks - those also lay on the freezer floor and block drain when you find a plate size ice on the floor, the unplug and thaw idea solves many problems - remember when our parents did this twice a year? You have not mentioned the defrost timer which stops the current flow to the compressor and leaves the fan/s on - much like the unplug thaw method. defrost timers can be cycled using a small screwdriver to advance the clock - the typical pump off time is every 8-10 hours, you can observe this using a killowatt meter on a short cord because seeing the outlet is awkward. when you advance the defrost timer the killowatt will show amperage drop during the defrost cycle, prolly about 1.5 - 2 amps for the fans, 7-10 when the compressor is re-activated.

Never had that problem, but I'll relay this one for others to learn...