I've read through your post and it seems like you have been very thorough checking out the wiring,I like what alan moore suggests about the dynamo it is obviously giving a good output but is it of the right polarity.
Have you checked out the earth wire from the mechanical regulator to earth, does it connect not just to the frame but to the battery earth as well,you could use a buzzer to check it,this wire helps pull in the relays in the reg box,if it was broken the regulator cutout wouldn't pull in, I checked through and you don't mention it you have F-field A ammeter D -dynamo armature E -earth
I am on the standard FADE approach in the standard MCR box ( I have two in fact)
I am pretty confident that I have checked all the wiring for resistance end to end. However, what I am going to do is to chuck in another dynamo and then do everything again, step by step.
Once I have eliminated the improbable, then the only thing left etc ..