As an indication of the oil return, try a transparent plastic pipe fitted onto the scavenge pipe dipped into oil in an oil drain pan under the open sump. A bit messy, but shows the relative balance between feed and return. Poor pick up is a combination of obstruction, air leaks, poor seal between pump and crankcase, or just a poorly performing pump.
Here look for a worn, cracked body, sheared off drive tab on scavenge gear, really badly worn or loose end plate. Not all or any may apply to your pump. Long shot, but substituting another pump would be well worth another try before a full strip down to change the cases.
The PRV is an enigma, as open or shut the total volume of oil delivered is the same, whether blown off or fed under pressure to the crank bush and bearings. But experience would indicate changing the PRV can sometimes effect a cure for mysterious oil problems.
Swarfy.