Many people put pumps before the injection pumps for multiple reasons. Several have stated one of the reasons, preventing air in the lines. A vaccum line will tend to pull air in through any fitting that is not 100% tight. A pressurized line (even a very low pressure) will tend to leak fluid out of the fitting, rather than pull air in. So that would solve one of your fears - fuel loss. Air bubbles equal fuel loss, albeit in a small quantity.
Supplying the pump with fuel, rather than making it suck it out of the tank, will always increase its effeciency, because the pump is not having to pump fuel out of the tank and to the injectors.
You are so concerned about a loss of fueling under load. The 100% obvious answer is to use another pump to supply fuel to the injection pump so it doesn't have to supply itself.
1st gen Cummins VE44 pump is a mechanical VE pump. Those trucks came stock with a pump supplying the pump. 'Nuff said.