Jeez, sorry to read that you have not resolved this issue as yet. You have proved that electrically the gauge is connected and responds by deflecting when you disconnect the terminal on the pressure sending unit... Perhaps the sending unit is faulty... I suspect the sending unit has an electrical resistance within it`s casing, and that resistance alters relative to the oil pressure applied to it by the pump.
I have no idea of what that resistance might be, though if you understand the operation of a measuring volts or resistance in ohms, then there are tests you might be interested in making.
On the other hand you might consider it worth a punt to buy a replacement oil pressure sender unit.
If you do not mind getting messy... try removing the sender unit, check that the orifice in it is not blocked in any way...which seems unlikely, but I live in hope... with the sender unit removed and spinning the engine over with the ign coil disconnected to prevent the engine from firing up... Ideally oil should have been spat out of the opened gallery...?
fingers crossed for you yet again.
