LED power management
a) NO buck converter is 100% efficient, 85% is more realistic.
0.66A/85% = 0.77A
c)Worst case scenario: Assume star ground connections with star point on the side of the grid.
Assuming 5m per row and 0.5mm2 wire we get 171 mOhms per row. Current will be dropping with every cell, thus voltage drop will be well below 100mV. If we take into account fact that each WS2812 is acting as amplifier you can ignore ground wire voltage drop.
d) Run one thick(2.5-4mm2) cable from PSU to the grid, then spit it into 11 rows.