1) It will be 3000psi regardless of pre-charge pressure.
2) No, if the pre-charge pressure is below limits the aircraft cannot legally be dispatched. If the accumulator was within limits then the aircraft could be dispatched as long as the second pressure gauge (accumulator/flight deck) was reading approx. 3000psi with hydraulic pump on.
3) MEL 32-13D for flight deck brake gauge. MEL 32-13C for wheel well brake accumulator gauge.