1: Outside of multipliers, all girls are equal.
2: That was from back when earnings and cost were different for every girl.
3: Doesn't really matter, in my opinion. Just prioritize the 11th girl, upgrading the others is just drops in the bucket until you start hitting the wall of your current multipliers.
4: I don't mess around with the stocks nearly enough to say. If I'm remembering right, someone here has used the stocks to reach beyond the point where girls have upgrades, so probably pretty good if you have the patience for it.