I think your losses are pessimistic. Even using about double the flow rate we were talking about to keep inlet temperature down (1GPM) you shouldn't need 50mH2O of head. A common 360mm radiator like the Swiftech MCR360-QP has a pressure drop of ~0.2mH2O at 1GPM. Even three of those in series would only be 0.6m of pressure drop.
I don't know how those blocks are internally constructed, but most common CPU waterblocks are 0.5-2mH2O @ 1GPM. If those two blocks in the C1 are close to the 10mH2O at that speed you would need in order to get a 50m total drop, I would be extremely surprised and they would be a terrible design.