Oil pump priming question

It could be a combination of issues!

Another possibility is #5 main cap bolt related. Did you use washers under the bolt heads and/or use aftermarket main cap bolts? I've has instances where a washer caused one of the bolt heads to interfere with the oil pump housing, keeping the mating surfaces from sealing. Solution was to grind down the head and/or the oil pump body for clearance.....or just leave off the washer on that one.