I wonder if some of those rigs are more fragile than the others. Perhaps some of them (rig (a) perhaps?) would wiggle more severely if the rowing forces were uneven while others would be fairly robust to differences in force.
I wouldn't expect so, at least by the math. It looks is pretty simple. Number the seats 1 to 8, then for each side sum the seat numbers. The standard rig gives 16 and 20, but all 4 of these solutions are 18, 18.
