The hose temp difference suggests that for one reason or another coolant is not passing through the heater properly. Whether that is because of a physical obstruction or an airlock, and whether an air lock could account for your observations, is a matter of speculation.
Isn’t the simple approach here to get the nose of the car as high as possible and burp the cooling system first and then, if that has no effect, to consider changing the heater matrix? The temperature gauge and its linearity or otherwise seems to be a red herring.
Is the air-blend control working OK?
I should say my own record on diagnosis of odd faults is not good.
“Coolant always flows through the heater core regardless of the position of the heater controls in the passenger compartment. It is very important that the heater core path from the rear of the head to the water pump inlet not be blocked, since otherwise hot coolant in the rear of the cylinder head will stagnate, making the rear of the head overheat.”
So the completely worn out water pump may not have been the only reason for the overheat 5 weeks ago.
+1 for the heater core flush with hosepipe. If you want to go one better drain the heater matrix with both hoses removed and refill with a pickling solution of 4 ozs citric acid powder dissolved in hot water. Leave for an hour or two and then flush with hosepipe.
If the heater core is restricted it may be that the relatively slow water pump at idle allows a heat soak effect at the matrix. When driving the under bonnet temps may be reduced and the fan is being fed with cooler air.
I agree, and I’d add that if there’s debris causing a blockage you may be able to flush it out by running the water backwards through the matrix. So run the water in through the pipe that connects to the hose leading forward under the exhaust manifold, and out of the pipe that usually gets fed from the back of the engine.
