My understanding that i-mode markup was very limited and didn't include tables or css, so I don't think it would be possible for desktop sites from 2011 to have been designed for i-mode first. Common Japanese sites for desktop browsers from this period as described in the original article were extremely cluttered in a way that would have been completely impossible to read on a feature phone with i-mode.

I believe that as with WAP, it was necessary to have completely separate i-mode sites, so I don't think the front end rendering of one would necessarily affect the other even if companies like Yahoo Japan were serving similar content over both.