Good new first: testing the injectors was the right thing to do and it turns out cylinder 1 injector (the one that was wet when removed) has a "taking a leak" spray pattern:
https://www.youtube.com/watch?v=MYt06n9Qv24
What are the odds with a freshly reconditioned injector?
I was pretty confident swapping this one out with a known working injector from the original batch would finally solve the issue, but of course it didn't. After reinstalling all injectors and fuel lines, hard start continued and I now can see air bubbles going through the transparent IP intake hose. They might have been there all along, but I honestly did not notice them till today.
I then went ahead to replace all leak-off pipes (again, since I originally reused the ones I had mounted earlier), followed by all copper o-rings on the injection pump (intake + out screw), followed by retightening all hoses on the filter and replacing (again) the plastic return valve. Nothing made a difference, bubbles are still showing up.
The engine is off the car, so the fuel lines are as simple as canister -> filter -> IP -> filter -> canister.. it should be an easy one to figure out, right?
Is there any trick to more easily locate the source of the leak? I know the back of the pump might also be a source, but the damn thing is like new.. at the very least it should be sealed. I cannot see any place where diesel is obviously leaking. I gave up for today, too exhausted..
@ngoma to your questions: yes the block was rehoned and all pistons (+ their rings) are new. Regarding IP, I got it off of eBay and the seller has nearly 100% positive feedback. I already had trouble with them though, since clearly they did not know how to properly set the cold start device. But I tend to believe the rest should be in order. Regarding the throttle lever, they would have done that too, so hard for me to tell whether that is a possibility.