Houston............. We have a problem! (No surprises there really).
I tried putting all the half-shafts, backplates and the 2WD unit in place as it will be in the final configuration.
Serious "binding" issues and on one side the half shaft just would not insert (despite the use of a very large "persuader").
It looks like something is scoring a line along the half-shaft as it is slid/pushed/hammered inwards.
It only takes the tinies bit of misalignment to totally kybosh it all it seems.
I found that despite almost getting there when the hub-shell reached the shoes it stopped totally.
This is a concentricity failure I think, so I will have to find out what has introduced it.
It may be as simple as a shim/washer to take a twist out of a backplate (I hope).
It does call into question the viability of the design because I would hate to try and pull these half-shafts out without a whole mess of tools around to attempt it.
Doing it at the roadside would be a nightmare I think.
Oh well, a lofty enterprise is still a good learning opportunity even if the outcome is not as one had hoped. We shall see, tomorrow is another day.