Alright then so I have swapped the coil and leads! Immediately, things were better. The massive, jerky hesitation was gone, however above 4k the car was still feeling sluggish - felt to me like it was missing a cylinder.
I took it for a big drive and gave it a bit of a spanking, and the hesitation has crept back. It's nowhere near as bad, but its there.
So, would I be right in assuming that maybe the coil that @axle117 extremely generously donated is on the fritz too, and as the car/coil got hot, it started to fail (how i'd expect a coil to fail normally)? I wondered if perhaps the plugs might have been a little glazed after all of the prior misfires, but since the failure appears to have come back a little, i'm guessing coil fault. Would that mirror everyone's thoughts?
Ill order a new one from my friends at Subaru Lower Hutt tomorrow if so - I REALLY appreciate everyones input on this. I'm so infuriated that the problem I initially thought it was on day one, that I've spent 6 months trying to diagnose, is the fault after all. You guys are absolute legends.