If you use auto level it automatically trims you for "level" flight at that speed. Turn off auto level and that trim stays. However, if you have a few rotary dials (depending on your stick setup) and you map them to the 3 trims, and set them, then when you turn off autolevel I think your trim axis controls take over again and you get your old trim back.
That's from memory on a similar question.
Combat trim in a p38 = BAD. Combat trim is always trying to trim you for level flight or something, so when you pull UP it's trimming you down, or dampening you, or something like that. It screws with the performance of several aircraft, the Ki84 and P38s included.
G is agreed to be the most nimble, but it's also the weakest-powered version as well. I think all 38s can dogfight equally, for the most part.
It should be possible to set up a twin throttle. I don't know how, as I just have a twisty stick analog MS Sidewinder.