I can help with a few of those....
The heater, there is a small lever on top of the centre tunnel in the floor, on the right hand side just in front of the seat base. This is the heater control. If closing this flap doesn't stop the hot air, then the rubber seal inside the flap will be damaged. If you lift the carpet there is an inspection hatch you can unscrew and get relatively easy access to replace the flap.
The idle speed is controlled by 2 screws on the carb. There is one on the throttle arm that sets the throttle stop position, and a screw very near the base of the carb facing the rear of the car which is the idle mixture. Raise the idle speed a bit with the throttle stop, then adjust the mixture screw til the engine runs fastest / happiest. Then you can reduce the speed a little with the throttle stop, then check the mixture is still correct. Repeat a couple of times till you're happy with the idle speed. Dont forget to check it idles happily with the lights/wipers on, when the generator is drawing power.
The stumbling at speed could be many things. As its a new car to you, a general once over, checking the spark plug condition, the timing, valve clearances and possibly even giving the carb a clean out might help. It's possible that the stumbling at speed could be related to the same weakness that is causing the poor hot idle.
Edit:
The door mirror I think will be an aftermarket one bolted to the original holes n the door, which means its low and useless... ask me how I know.. haha.
The ignition positions are centre = off, clockwise = ignition on and anti-clockwise = lights only, so you can leave the car with parking lights on. Be aware it puts the whole lighting circuit on though, not just the side lights, so you'll need to use the column switch to select side lights should you want to do this.
There is a user manual for 500L in the downloads section that you might find useful, even though your car is an R it will largely still apply, it helped me to know what all the switches did and things on my F!