[Blackdog_kt]

Whoarmonger got most of it and you seem to have a good grasp on things, so i'll only add two things:

Mixture: Not all people agree it's a bug, because it was reversed in the real aircraft too. This is a carry-over from manual mixture systems, so that by pulling the throttle back the detent on the throttle lever will also pull the mixture back into rich to prevent fuel starvation.

Try to think in terms of what you see the controls doing in the cockpit and not what your key bindings are titled and you'll be fine: auto-rich mixture is with the lever fully back and auto-lean with the lever fully forward.

Prop pitch: Think of it like gears in a car. The two-stage prop only gives you two gears (fine and coarse pitch) so it's like you only have 1st and 4th gear, but they do have a pretty wide power band. In fact i find these Spits pretty easy to manage compared to a 109 which has fully adjustable but still manual pitch.

Use fine pitch just like you would use 1st gear in your car: Slow climbs, low speeds, to limit your speed in a dive (just throttle back a bit to avoid over-revving it), when you want faster response to throttle changes (both decrease and increase) like for example when taking off/landing. It's good to have it on fine for the landing approach in case you need to go around too.

Use fine pitch like a 4th gear: Fuel economy during cruise, attaining your top speed, accelerating in the dive and climbing while at high speed (switching to fine for every climb is like driving on the highway at 140km/h and downshifting to 2nd gear whenever you see a small upwards slope on the road: the torque sure is enough to climb it, but you are bleeding off speed like crazy).

Keep in mind that changing the pitch will also affect manifold pressure, even more so with these props because the pitch change and resulting torque change is quite big: boost will rise when setting coarse pitch so you might need to throttle back a bit, in a similar fashion it will drop when going to fine pitch and you might need to throttle up a bit.

As you can see, it's mostly situational and there's not a hard and fast rule. In fact, there are cases when flying at fine pitch all the time severely limits your performance, like for example trying to catch a 109 in a dive: fine pitch will not only cause the RPMs to pick up and force you to throttle back, it also gets to a point during the dive that the prop creates considerable drag, due to the prop blades being almost face-on to the incoming airflow. It's like having a disc-shaped airbrake in front of you.

Switching to coarse pitch in such a case presents a smaller "head-on" cross section of prop blades in regards to oncoming airflow and lets you pick up more speed.