Proportional pressure controller and gas flow control

Yes, you can use a proportional pressure controller to control the pressure. Controlling the prssure will automaticaly controll the flow through a constant size orifice.

yes back pressure will also affect the flow through the orifice the flow gets minimized as back up pressure builds.

Why is the orifice there? A pressure controller controls pressure, not flow, and flow and pressure are inter-related, especially if there is a changing pressure at the end of the pipe downstream of a pressure regulating device; one could stop the flow completely and the pressure would remain constant.

Consider the use of a needle valve and a variable area flowmeter as a possible alternative solution, like on the cylinders of argon used in inert gas welding set-ups, especially where the set-up is manned, perhaps?