My first question is: is the mid point at 90 degrees the optimal temperature for the car to be running at. Or the max temperature where the cooling decides to work harder and bring it down if it goes over?
Or is this the same thing?
The system is designed to run more or less in the middle. Most gasoline cars are around 90-100 degrees Celsius. The thermostat in the cooling system determines when the radiator receives coolant flow and thus when the temperature starts to level off. So, yes, it's pretty much the same thing. But thermostats typically open at 80 degrees or so, the system just levels off a little bit higher.
Fun side note: On some cars (early Miata oil pressure gauge comes to mind), they even made the gauges really dumb. If there was oil pressure at all, it told the gauge to go to the middle. Otherwise, you didn't have oil pressure. Apparently they had people coming in to service because the gauge moved too much. Could also just be a rumor. Still, I wouldn't be surprised if temperature gauges were made somewhat dumbed down, but I digress...
I also noticed when it's very cold I put the heating on full heat and max fan, the engine temp takes much longer (15 mins or more) to get to 90, as no doubt I'm leeching heat from the engine into the cabin.
Possibly. Are you comparing to the starting times when it is cold out but you don't turn on the heat to when you do turn on the heat? Or just when cold out in general? Because the engine's going to take noticeable longer to warm up in the cold regardless. Some HVAC systems also prevent the coolant flowing to the heater core until the engine is at some predetermined temperature, so it may have no effect. But, some don't prevent that so you can 'leech' from the coolant system.
But why are you blowing cold air at yourself?
I have heard that to work optimally catalytic converters need to get hot. By turning the heating to full and so delaying the engine reaching that 90 degrees and I burning fuel less efficiently / cleanly?
If the engine is cold, yes. This is where manufacturers are currently focusing their emissions cutting efforts. Cold-start emissions are the worst, as the car requires significantly more fuel when cold, in addition to the catalytic converter not being up to temperature.
This also touches on your first question. The warmer the engine, the more efficient it is. I don't know why ~100 C is where most cars (not sure about diesels) sit. I imagine it was pretty much the limit of easily available engine coolant with good 'antboil' properties, among other things (oil temperatures, etc.). That's actually something I'd be interested in learning.
I have heard that running the AC burns additional fuel (or takes power from the engine that would otherwise go to the wheels). Is this the case for heating, or just for cooling?
Yeah, it does but it's not very significant. The system will be removing moisture no matter what the temperature setting is.