Let me know if I've got this right.

Page 159 says 'At the end of each Initiative Pass the gamemaster subtracts 10 from all the characters Initiative Score.'

Lets say a character with an Initiative Score of 5 takes a complex action and then the combat moves on to another character's Action Phase, but still in the same Initiative Pass. The character could then declare an Interrupt Action (costing 5) and reducing their Initiative to zero. But if the same character took the same Interrupt Action first they wouldn’t have been able to take their action as the cost of the Interrupt Action reduces the Initiative Score to 0 immediately. Is that correct?

As far as I can tell, yep.

That's the tall and short of it.

Ok, cool. Just needed a second opinion on that, my brain kept telling me it wasn't quite right.