Hey whatnot;
I will leave 1) to others as the answer depends hugely on the aircraft... There are as many tips and tricks as there are pilots and planes. However yes the supercharger stage switch is automated in P-51s.
2) Minimap path on. It will however show your own plane on the map as well. Many "full real" servers leave it on. Map, stopwatch and compass are underrated...
3) I typically spot cons around 10-ish km or a side of a map grid away if they are co-alt, the higher angle up or especially down, against other than water, the more difficult it is to me. I would suggest you to develop a searching pattern, mine is paranoid looking to all directions and zig-zagging 20-ish degrees every 10 sec to check every angle. For ID-ing spotted contacts, further away, see what they are doing: where are they, where flying, climbing, diving or level, flying towards/away/formation with other contacts? Closer, you can use the closest FOV view to be 100% positive, good things to check are general colour, position of wing(low, middle), its shape(elliptic wings can be distinguished from straight ones very far!). Inline or radial engined? Typically the game is axis vs. allies with very limited planes - if you know, for example, that the other side has just Spitfires and an odd hurricane, a gray or green radial engined, low-wing straight-wing fighter you just passed is definately not a brownish inline engine elliptic wing Spit, and probably not Hurricane.
BTW never chase the contacts, unidentified or 100% sure bandits. In full switch theres a good chance its a trap, and your contact is letting you close in purpose...