2013 Temp Warning Driving over 50 MPH

I have a 2013 III with 230k miles.
Over the weekend, the engine temp warning, then check ingine light would come on. I noticed that if I kept the speed under about 45mph, and drove mostly on battery, the temp light remained of. It only came on at freeway speeds or climbing gentle hills.
I had to add about 1 gallon of coolant in driving about 400 miles.
There does not appear to be a leak, only coolant sprayed in the engine compartment from the reservoir. I have not replaced the original spark plugs , and just changed the oil and transaxle oil today... the mechanic I spoke with today, said coolant was probably getting into the engine. I don’t think he gave it a thorough inspection, though. Any ideas on possible reasons? My thoughts of possible causes are;
-bad plugs
-water pump
I’m taking it to the dealer tomorrow for plugs, so am interested in any help/suggestions.

 

I get the red thermometer, then the yellow check engine. The temp light then goes out shortly after the check engine comes in.

Also, shortly before the temp came on the first time, there was a knocking or tapping or clicking sound that came from the right side of the engine area. That happened a couple times, with about 6-10 clicking/tapping sounds each time. I haven’t heard it since. This led me to believe it might be a failed impeller or water pump.

Would a bad head gasket cause coolant to come out of the pressure release in the coolant reservoir? because that is where at least some of the coolant is going. It’s been sprayed around that corner of the engine compartment (around the reservoir tank).

 

No white smoke, and coolant does not seem to be coming from anywhere except the reservoir pressure release. No real odd sounds, although the engine did seem to be a little louder than normal while accelerating to freeway speeds.

The oil appears normal.

 

Fuuuggh...