Have essentially checked or replaced all the ‘normal’ stuff, are well out of ‘the box’!!
edit
Am so far out of the box to be in the warehouse next door!
Is there a way to narrow it down? Close the outlet on the pump, & see if it makes pressure?
4 reasons an oil pump won't build pressure:
It isn't rotating.
It isn't priming. Air doesn't pump really well.
Pump doesn't pump.
Not enough restriction in all the combined passages to build pressure.
I'm not there, I'm only half a mechanic, but I think it'd be helpful to narrow it down to what is causing the failure.
It occurs to me that engine design wants oil to flow, passages are sized to provide flow everywhere. Open one path too big, (as in a bad bearing) you lose pressure everywhere. While volume of an oil pump is reduced if pumping only air, the ability to build pressure is also reduced. Air flows easier than oil.
Is there a way to tap the system on the pressure side of the pump & send oil in by other pressure source.
Again, outside the box here, I might try a retired propane cylinder, a nipple & T. Through the T, install a dip tube plumbed to the oil pressure sensor tapping, or what have you on your engine. Tap in the T gets a Schrader valve to pressurize the propane tank. Funnel the tank full of oil. Feed it through the engine.
It'll give you some information whether your engine is wasting flow, or if pump isn't providing flow. It'll also flood every bearing surface with oil it likely wants about now.
By the way, I have a few of these rigs to put gear oil into transmissions, differentials, etc. They work for oil undercoating (rustproofing), solvent washing, etc.