[Baffler]

There's a lot going on there, Callum, so without seeing or hearing the car it really is rather difficult to diagnose. In saying that the first thing that springs to my mind is a duff Engine Coolant Temp sensor, therefore, the ECU thinks the engine is cold all the time so is chucking fuel into the cylinders which leads to unburnt fuel entering the exhaust hence why your cat got so hot. A duff ECT will dramatically affect the idle and send incorrect values back to the ECU hence your fan coming on.

My first task would be to test the ECT sensor and its wiring, replace if necessary and clean the Idle Air Control Valve and Throttle Body butterfly valve for good measure. All the "how to's" are here just do a search for same.

You mention the previous owner of 17 years replaced just about everything but were these components replaced with new genuine Volvo items, reputable OEM replacements or the cheapest option possible?

I should also state that if you intend on running two of these beasts I suggest you invest in a copy if VIDA, familiarise yourself with how this engine and its components function, and also a DICE interface in order to communicate with the vehicle.

Oh and congratulations on the new arrival! Hope mother and baby are doing well!