Pretty normal althouh mpg does sound a bit low - flat spots might be AFM resistive track wearing but might be a lot of other things as well!

The FPR controls the pressure to 3 bar (45 p.s.i.) above the inlet manifold presure - that's what the pipe connecting it to the inlet manifold is for. The idea is to make the pressure drop across the injectors constant.

>The fuel pressure is mostly at 36psi.
With the throttle closed the inlet manifold pressure is less than atmospheric hence if you add 45, to to say -10, you would get 36.
>Under hard acceleration it peaks briefly at 44psi.
With the throttle wide open the inlet manifold will be close to atmospheric hence fuel pressure will be 45.
>When I clamp the return line it goes to 75psi, whoa.
Don't do this, you risk damaging the fuel pump and or blowing a connection.
The Bentley spec is 40.6 ->46.4psi on a running engine.

>Is 4psi (10%) significant? Will I see any improvement in smoothness or MPG with a new FPR?

I think your FPR is spot on!