Here is a description from the physics model programmer:
It is normal for MP to overshoot the default target of 29.6“ on takeoff because the MP controller is an inert device and depends on engine oil pressure to close the turbocharger wastegates. When oil temperature is lower, as usually on takeoff if not thoroughly warmed up to ~180F before, oil pressure is higher and the MP controller target point shifts above 29,6“ (higher pressure closing the wastegate). It is indeed highly sensitive to oil temperature changes and will be unacceptably high below 165F. As the manual suggests it is acceptable for MP to rise into the caution range for 2 minutes on takeoff as you can be sure about oil temperature to rise quickly after takeoff.
The Manifold Pressure Target slider indeed replicates a setscrew a real mechanic has to adjust the manifold pressure target at 180F oil temperature, it can be used e.g. to tune a particular engine‘s full power output down if CHTs are unacceptably high. The Fuel Flow at MP Target slider adjusts another setscrew to control fuel flow at full throttle and 29,6“ of MP. Again this can be used to control engine temperatures on a particular engine or power output, as e.g. on very hot days with thin air the fuel flow should probably be slightly lower to avoid too rich mixtures at full throttle. As a summary, the sliders are probably more interesting for real aircraft owners to tune the simulated engine to the parameters of their real one.