joejack951 RobbieTunes Thank you for the help on this!
There was a small amount of corrosion inside the fork steerer tube, but none close enough to the thread end that I could feel it with my index finger (almost twice as long as the section that goes inside the steerer). I cleaned it out with a rust remover (that I have used to great effect on the same frame) and a rag anyway. I also ran a threaded headset locknut over the threads and found no roughness at the level that the innicycle headset was binding at. There was a little a bit lower but that was sorted out.
It is now turning 4 1/2 turns (1 more than it was, I got it wrong in the original post) and binding at the same spot every rotation. The binding gets worse as it goes in further, to the point where I can't turn it anymore and I am worried about damaging the aluminium threads in the headset (I haven't used a stem for extra leverage yet).
...now for the good bit ...
joejack951 I think you are right that the problem is a concentric steerer tube. The wall of the steerer tube looks slightly thinner on one side, though it's hard to tell because of the thread starting point ... however calipers confirm this hypothesis ... the wall of the steerer tube ranges in thickness from 1.58-1.96mm! Do you think this is enough to make a difference to the fit, or do you reckon that the problem lies elsewhere?
Thanks again for the help!