3. While by no means an expert, judging by the sights in German planes being offset to the right of the centerline, I would assume they were meant to be used with both eyes open, but only looking through them with one eye, the same way reflex sights on modern rifles are being used today.
I agree it is somewhat of a hassle for people whose left eyes are dominant. I know the first couple of rounds during gunnery training I always spend trying to switch my eye dominance around - if I don't, all my rounds will actually pass my chosen targets to the left.
4. I would assume it has something to do with either noise or exhaust entering the cockpit.
6. The MG131 did in fact have access to HEI ammunition. I don't know offhand what the ammunition mix is for it ingame, and I know the ammo choice for the P-51's M2s has been...controversial, but pound for pound, the German 13mm was among the best HMGs of the war and certainly better than the Browning.
Note I said pound for pound, not gun for gun. One M2 is certainly superior to one MG131.