Yes as you imagined, a leaking IP input shaft seal can cause a no-start or hard start condition by allowing air intrusion. But your temporary
low pressure electric fuel pump should really negate the chance for air intrusion and allow it to start if it's going to.
(Key is low pressure (3-5 PSI) fuel pump. Don't want to blow the rest of the IP seals.)
Place a catch can under the leaking IP.
Let's see, you've tested and have 6 good GPs, the GP system is working as designed, you've poured MMO into the cylinders, checked the valve clearances, verified the IP timing is at least in the ballpark-- Other forumers, help us out: What else are we missing here?
Question for you:
How tight is the IP belt? Should be able to easily twist it 45-90deg using just thumb and finger.
Too tight and it wears the bushing which will cause constant premature seal failures.