I hadn't looked at your diagram or the discussion behind it before, and nothing I've said is aimed at it. Your engineer friend is right of course, but if sufficient volume and pressure is available then manifold design becomes a little less critical (within reason). Some pressure differential or imbalance is likely acceptable in operation. The extent of that imbalance will only be worsened by things like cam and rocker feeds and during marginal adverse situations those flow/pressure differences due to manifold imbalance will likely determine what fails first.
So, I would tend to agree that larger main oil galley size would benefit the oiling of these engines more than anything. Increasing the passage sizes from the pickup all the way through the filter circuit and main feed back into the block would probably reap a majority of improvements available in the factory oiling scheme. Maximizing everything from the pickup to the main while minimizing (without over-restricting) everything past the mains should yield success in almost every case. Until the rods run out of oil at high revs..
I also agree that oil doesn't need to 'turn' to get to #4 - oil will 'expand' into any area where pressure is lowered. There is some momentum due to oil having mass, but it's not solid particles and it's also not a compressible fluid like air..
