Rukia's execution was accelerated but you forgot about when Yoruichi said that when they went into the Soul Society they went through some type of time warp. Where it would've taken them 4 days to get through the soul changer thing they arrived the same day. Giving them 6 days when they started training to achieve Bankai. When they moved the execution up, they moved it to the 3rd day when he was supposed to be done.
2.He achieved BanKai in 1 1/2 Days(remember Rukia's sentence was accelerated)