Yamamoto's shikai attacks disintegrate things to ash. His bankai is that but over kill. Also the chichibukai are at a disadvantage seeing as yama-ji can also summon those flame spirits giving them even more numbers and a distraction. Yama can just flash step at the same speed he moved to get to the quincies and reduce all of them too ash.
Also yama can use his bankai in both scenarios considering he has always had it~ Please refrain from saying some crap about proof because he wouldn't be captain over all of them without it. And also i forget where but i think juha bach said he had seen it before and the other captains when their lips got chapped new he had released so they obviously knew it existed prior to that.