I am not sure of that particlular pressure gauge set-up. The Benz test port uses the same fitting as the R12 a/c charge ports.
Any way you chose to check for either spark or power to injectors will indicate OVP , as OVP is power source for ECU , which controls both ign and fuel injectors , but not fuel pressure, That is FP relay, but you already seem to know that.
I think you will find upgrading the OVP will solve the problem
Good Luck