[inigma]

phonetics knows no spelling style.

Elven and elvish are two totally different sounding phonograms.

Elven can be spelled Elfin, and elvish can be spelled elfish. That is not in dispute.

The difference, I think, is in phonetics, and the "en" sounding ending is meant to imply a straight relationship; whereas the "ish" sounding ending is meant to imply associative relationship.

To prove my point: you can have Elvenish, but never elvishen.