No cabin heat. Overheat icon flashing.

There is another coolant sensor on the exhaust gas heat exchanger circuit, but since you have that bypassed - that shouldn't trigger an over-temp event. The gas flapper valve would get stuck closed, causing extra hot coolant to enter the cooling system - the over-temp lamp would trigger, but if you don't use a scan tool, you wouldn't know that the exhaust gas coolant sensor was triggering the over-temp lamp; NOT that the actual engine was over-temping.

 

If the OP is no longer getting overheat warnings, I'd suggest looking into the hvac control. I had a similar issue with a c-max energi about a decade ago but it would only happen in hybrid mode and only on the passenger side. A new blend door controller fixed it. There were no codes or icons flashing.

The OP's overheat issue could have come from a botched fix attempt (air in system) by the PO chasing the wrong fix solution for no hot air.

If it's not too cold out, the OP can try to keep it in ev mode and see if the electric heat system functions.

YMMV