I thing Mencken overlooks a valuable distinction here.

"Our now universal use of you for ye in the nominative shows the dative and accusative swallowing the nominative, and the practical disappearance of hither, thither and whither, whose place is now taken by here, there and where, shows a contrary process. "

The words ending the "-her" imply motion in given direction.
"her" is an adverb in German, used to indicate direction.