re 91u JIS layout: are the Win keys on that model in the same place as the 86u?
I can certainly see the need to compromise somewhere, and some people will be disappointed for sure, but as I see it this case is exceptional, so 'cheap' solutions are maybe not the way to go.
Noted the comments on popularity of 86u, is it even available to buy new anywhere? I think if the 91u doesn't line up with the 86u, then I would maybe lean toward building the WKL design around the 87u/88u as those are the keyboards that are available.