1, Everyone probably has their own idea on this one, there are probably nearly as many definitions of kill stealing as what constitutes a head-on attack. Personally I'd probably take the shot.
2. What you're looking for in a base defense plane is something with good acceleration, a plane that builds E quickly and is at least reasonably manuverable at low speeds. I've been playing long enough that I pretty much use my main ride (P-38) for everything, but I wouldn't say it's the ideal choice. I would say the 109F you used in your other post would be a decent choice, manuverable, good acceleration, good climb. Only downside is top speed and firepower in LW which doesnt matter IMHO as much in that scenario. Other planes I'd consider would be the La-5/7, N1K, and Spit families (probably the V or XIII), I used to enjoy using the La-5 from time to time for base defense before I became a crazed P-38 jockey.
In general you want to build up some speed before you tackle them, speed will give you options going into the fight, I'd put the base between myself and the town, climb up to 1-3k and build up some speed (in the 38 I like to have 275-30mph or more, indicated air speed before engaging. By putting the base between you, you force the bad guys to either fly through the ack or around it which buys you time to build up for energy state before you have to fight.