Your logic seems fine to me, but as none of us have managed to solve this one, perhaps having the same thoughts isn't a good thing !
The only thing that's crossed my mind is that when I say mine gets pressure on cranking I am using a 0/40 oil and not the original 20/50 which I know many folks prefer. That should make getting pressure easier and perhaps that's the reason I'm used to seeing the needle move. I don't know what viscosity you've got, but if you've not filled the sump yet and were going to use 20/50 then I would be tempted to put in a cheap 0/40 to see if the lower cold viscosity manages to get oil flowing around. You can always change it early if you're not happy with it, especially if it's cheap stuff.
When you remove the oil gauge feed, to follow the logic you've got 40psi oil taking the easiest route into fresh air so I would have expected a proper squirt, a few inches at least. The comparison with a known oil system that's working is a good idea. (is that why you've got so many Europas ?
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Otherwise, as said before, I'm stumped. I've attached the lubrication system diagram as I don't think it's given in the later manuals and that shows the oil flow around the engine, high & low pressure feeds and returns.
To get no pressure at all when the pump is known to be good and you've changed oil/filter/pump gives me a list of either an air leak on the suction side between the gauze & pump inlet so the pump lobes never fill properly, a blocked filter (but you've changed them) or a massive leak somewhere in the block, almost like a missing shell or thrust washer.
I missed the start of this saga so I'll just ask if you've ever had this engine running with good oil pressure or is it the first start up since you bought it ? The comments in an earlier post about mayonnaise in the sump make me think it's not a fresh rebuild ?
Brian