Donald is correct. At low speed, as long as you've got 20 gpm to the manifold, the system will operate.

Running on low speed, as long as it is sufficient flow to activate the flow switch, will not require running the AP on High Power (Cell Power 3). In fact it's the opposite. You should be able to set it on Cell Power 1 and just make adjustments to the Purifier Output % setting so that your chlorine level is maintained.
Plumbing the manifold should be so that the manifold base is at the bottom and the loop is up. You can mount is sideways too, but never upside down, where the loop is below horizontal, where the manifold base it.
Going with a larger size cell is what I would recommend as it does provide longer cell life, but also, more chlorine production in a shorter period of time, in the event that you have to run on high speed to get the manifold flow switch activated.

Hope this helps,