1. No, the sticky goo is nearly impossible to remove. My guess is somebody overheated the motor, the heat gasket started to fail, and the previous owner bought some SilverSeal or similar head gasket sealer. That stuff goes into the radiator in an attempt to seal a head gasket. It basically turns to glass in the heat of combustion. My friend had to use this stuff in an attempt to save the motor in his truck, and we spent several hours trying to flush it. The thermostat was similarly plugged with the junk. We ran a garden hose with flush kit for hours, reved the motor near redline trying to pump that junk out, and it just kept spewing out. It coats everything in the motor, radiator, water pump - EVERYTHING, and sticks to it like peanut butter sticks to the roof of your mouth. We gave up. he's gonna sell the truck with the understanding the motor needs fully rebuilt.
2. yes the radiator should be pumping out lots of heat. its clogged with that goo. you can try flushing the crap out of it by directly hooking it to a hose, but my guess is you'll just end up needing to replace the entire heater core to get maximum efficiency.
3. the CEL was likely due to the overheat condition. disconnect the battery and the CEL will clear. would be good to know what the code is, in case its something more serious.
Also, I wouldn't use anything Prestone in a Jeep (or Ford depending on year). Jeep's now spec HOAT antifreeze, like Zerex GO5. All newer Ford's also use HOAT. This is what you want to run. Prestone does not make an HOAT antifreeze.
The other possibility is the goo is Dex-Cool contaimination. Basically, you'll need all new radiator, new heater core, new water pump, all new hoses and to flush the remainder of the system the best as possible. If you can't get it out of the block & heads, you're looking at a new motor. If you've got a warranty from the place you bought it, this would be a good time to take it back and use that warranty.