The inverted Vee engine is the same as a Vee engine, but turned upside down, so that the crank sits at the top and the heads at the bottom. The thory is, for single engined aircraft at least, that the inverted Vee allows the pilot a better view over the nose. If you look at a Spitfire you can see the cam covers make the top of the cowling wider at the front, particularly on Griffon engined models. If the engine was inverted then the top of the cowling coud be narrower, theoretically giving the pilot a better view.
The P-51D air scoop as little to do with fuel injection or lack thereof. The belly scoop is purely for the radiator group, while the chin scoop feeds air back to the engine.
Fuel injection is far superior to carburettors now because of the sophisticated electronic managment systems that are used. But in the time of WW2 injection was all mechanically operated.
The German engines, Daimler Benz's at least, used direct fuel injection - fuel is injected directly into the cylinder. This has the advantage of the supercharger only having to compress the intake air, and not the fuel. But it has the disadvantage that the effect of the fuel vapourising in the intake system is lost, and thus the intake air is hotter.
Some US and British aircraft used fuel injection, but I believe that these injected fuel into the eye of the supercharger impeller. In this instance the injection offers better metering with the cooling effect of the fuel, and is otherwise the same as a carburettor.
Direct injection systems also reduce the possiblity of backfires in the engine, as there is no fuel/air mix in the intake ducts.
Comapred to earlier carburettors, fuel injection was able to deal with negative G manouevres better. Egines fitted with earlier carburettors would cut out when negative Gs were encountered - something the Germans would exploit in the battle of Britain. Later carburettors solved this problem, however.
