'94 LX 1.9L: Personally wouldn't touch DexCool with a 3,000 mile pole. Also wouldn't go back to that store either.
For the first 2 or 3 years, I didn't even know my vehicle had a 3-speed fan. It mostly stayed off and when it did run, was on low; and then only ran for a short time. Later I found out it had a higher speed, and was surprised to find it had third speed.
The question is, did your fan run on high when the needle progressed high ? Was it really hot under the hood ? If so, you're readouts are probably precise.
I don't have any experience with your car so I can't say what might've been going on.
The reason you're probably observing cooler running now, could be because the prophylactic thermostat bypass valve (if it has one,) in the water outlet, has (permanently) opened.
I observed similar wild temperature swings when the headgasket and/or head was leaking. Other than occasional rough (but not really rough) running and slowly disappearing coolant there weren't really any obvious symptoms in the beginning. Much (much) later the coolant did start to discolor. (and a film appear on the inside of the windshield) Fortunately compression tests are pretty easy to do.