Is fasting the ninth day sufficient for someone who drank after Fajr and did not form the intention to fast beforehand, or should they fast the eleventh day instead?

Your fast for this day is not valid as long as you have consumed something that breaks the fast; therefore, you are not obligated to complete it. However, if you had not consumed anything that breaks the fast, then it is permissible to intend to fast this day according to the majority of scholars who do not stipulate the intention to be made the night before for voluntary fasts. This was narrated from a number of Companions and Tabi'un, according to Ibn Qudamah in Al-Mughni.