I have to have 15V on the proto shield, to be able to interface outward, regardless of a switch mode or other type of power supply?? It is already coming (at 15V) from the external supply, which does happen to be a switched mode supply. Not sure what that has to do with needing 5V for the Arduino itself, other than I'm trying to avoid multiple supplies.
Is it going to be a good idea to put a regulator on the proto shield when there's one on the mega as it is? The only reason I'm doing the drop from 15V to 9.1V is so that I feed that regulator something that's not going to make it so stinking hot (especially with a shield over it); I'm not trying to use a zener instead of a regulator. And I'm only using a resistor there as a current limiter; I'm not trying to feed the regulator from just a resistive voltage divider.
It just seemed weird to me that the mega would be drawing 20 mA, and I'm trying to be sure that makes sense before I change the circuit, in case there's something else I'm doing wrong causing that kind of current draw.
So maybe it would be better if I'd just asked "is it normal for the regulator on Vin for an Arduino 2560 to draw 20mA", so we don't get off in the weeds of telling me to use a different power supply? I don't see anything telling me the expected current DRAW from the board, only its ability to supply 50mA.