Why not positional variation?

[MarcoP]

That's a great (if depressing) summary of the state of the art.

I am afraid that the best compromise between "historically plausible" and "compatible with the text" is some form of automatic writing, as discussed by Gaskell and Bowern in their Malta paper You are not allowed to view links. Register or Login to view..

The other options are both poor fits for the text (e.g. how to explain Patrick Feaster's and Tavi's recent findings?) and horribly difficult to investigate or just formulate, because the number of variables and unknowns is huge.