According to a review on Square Sound (http://www.squaresound.com/final-fantasy-vi-piano-collections-cd-p44.html) the arranger is Reiko Nomura, who, as far as I've gathered, performed the pieces.
RPGfan claims that the arranger is Shirou Satou (http://rpgfan.com/soundtracks/ff6piano/index.php). That seems plausible, because he arranged the FF IV & V piano pieces.

There actually isn't any arranger credited in the original book or the reissue, it just says music composed and produced by Uematsu, performed by Nomura. There isn't any mention of Sato, and I'd absolutely doubt that he did it both because of that and because the style is completely different from all of his other arrangements. One might think the credits would point to Uematsu arranging it himself, but I'm also doubtful of that because of the style. Taking an educated guess, I'd say Nomura is responsible. She can play the hell out of it, that's for sure!

i always thought it was uematsu. because it is so melody focused with not much added (comparing it to the ones after 6 piano collection).

im not sure they would ask the pianist to arrange it as well as play it.