Ah, good memories. Other solutions to the lack of clearance between the rollover bar cross member and the shoulder harness bolt include putting the bolt in upside down with a nut on top, or cutting the bolt shorter. I went for the latter. Funny that the build manual just doesn't say put the harness in before the bar, or for that matter Caterham could just put shorter bolts in the fastener pack.
I wonder about the De Dion tube centering, exactly how critical it is. Like you I went to the effort to get it within a washer, but I swear it moves a little back and forth. I recently measured again, determined it must have moved, disassembled and reassembled, and wound up with the same number of washers on both sides. Maybe I'm just losing my mind, I dunno, but I suspect there is some play.
Thanks for the updates. If you can swing it those of us living vicariously would love pictures too!