I've had two IP's get stuck metering valves from a dry reservoir. Justin from R&D taught me himself how to adjust the metering valve because this type of thing can happen. It is "unlikely" as you say, but I'm assuming what the OP is telling us is correct, then his symptoms are nearly identical to when I've experienced a stuck-closed metering valve in an IDI IP.
I had this happen a few times. I had an IP on the shelf, installed it, and I was getting good pressure to the IP with the fuel lines fully purged. I even got diesel flowing out of the top IP fuel return, but nothing to the injectors. On a different occasion, I accidentally ran the tank dry (back before I fixed the fuel gauge) and it put air in the system. The dry IP reservoir allowed to the metering valve (this was on a 2nd IP) to stuck in a nearly-closed position, thus allowing a tiny trickle to the injectors, even though the fuel lines up the IP had been completely purged of air. The IP return showed fuel flowing as well.
I opened the IP in both situations (as per Justin's instructions over the phone) and fiddled with the valve (much like the video I posted above shows). Just by moving the action back and forth a few times, it freed up the valve and I reinstalled the IP cover. Then I cracked the injector lines and fuel flowed MUCH better. Within a few cranks, the air was purged and the engine ran.
This could be his problem, however, I'm not completely convinced he purged the air from his fuel lines. Opening the IP should be the LAST thing the OP does when diagnosing this problem. The up-to-IP-inlet fuel system should be completely checked over (
like in my link) before resorting to the metering valve manipulation. And the OP should take a few sessions of cranking before assuming the air in the injector lines are fully purged of air. Also, I'd like to hear from him if he pulled the IP return hose off and cranked it over. Is he getting fuel from the IP return line, but no fuel from the cracked injector lines? That would be a good way to help diagnose.