hard or soft copper line for a valley oil gallery bypass line?

Newbomb Turk said
BTW. your assumption that the pressure from each feed is the same. It's not. Also consider the FACT that somewhere in that pipe there are two different columns of oil meeting. That point can move back and forth in the pipe as the local pressure changes. Pressure and no flow. It happens.

You said you have a friend "who knows". Ask him which passage or passages from the main gallery 1-4 are going to receive no flow because the gallery is being fed from opposing ends. Maybe he can explain why this happens.