Do pump stacks transmit power vertically in both directions?

Solution 1:

Yes.

If you've got things correctly stacked, it doesn't matter where you connect the power supply, because they're all stacked, and if they're stacked correctly, they are effectively only a single "machine"; Your should be able to query any pump on the stack and get the total power needed (generally 20 x Number of Screw Pumps + inefficiency from gears / axles (5 or 1 per tile, respectively)).