The power reactor needs 1 z-level full of water, and a U shape transfers the water movement better than a V shape, really. I think I actually go with O-shapes, pouring the water straight back down to the lower z-level past the pump.
You'll need more than just two waterwheels for your 20 z-level pump stack. Expand the power reactor with 1 waterwheel at each side, and you should be golden.
Yes, you can put the gear below the lowest of the pumps, but I think there were some sort of issues with this leading to a situation with a gear ending up where a wall should be (to avoid the magma leaking all over the place).
What I do when I want to connect the power from a random z-level to my pump stack is the following:
z = x
#####
#+++####
#+++SsC#
#+++#+#
#####o========oPowerSource
####X#
z = x+1
#########
#####+++#
##CsS+++#
###sS+++#
###sSS###
####Xs
o = gear
=== = a horizontal axle
sS = a screw pump. S is the impassable tile
sS = a screw pump placed over a channeled tile
These are 3 additional pumps, but they contain the magma safely in the 3x3 room. Power comes from below.
It is quite possible that I have been overthinking this and that it is unnecessarily complicated.
Take extraordinary care not to connect the lever to a gear from which your pump stack is hanging in the air, else everything disconnects and !!FUN!! might ensue.