Half treasure sounds good to me. I'd almost be willing to say CR 11 based on how crippling limited mobility can be, but if you guys agree to 12, I won't argue the point.
Should we add a special "Advanced Faerie Phiz" section that notes with each X additional HD, the phiz gains one level of innate spellcasting in both druid and wizard, as well as +1 to its spell resistance?