2) Not all captains are the same. E.g. The older ones are much more powerful than your average captain. This is why Kyouraku was able to defeat the primera espada without even releasing his bankai.
As for Ichimaru, by all accounts he should be freakishly strong. Picture this: A true prodigy like Hitsugaya but with many more years of practice and experience + he's been guided from the start by one of the most powerful shinigami ever, Aizen. (I hope Kubo Tite acknowledges this and has Ichigo lose this fight like he's supposed to. Although it will probably end due to interference.)
3) What ichigo really is is unknown at the moment. He's a hybrid of a shinigami (from his dad) and an as-of-yet unknown being (from his mother).
4) Rangiku is not a zombie, she's just been hurt badly (Allon took out pretty much half of her midriff and Orihime isn't around to instant-heal people). She's looking for Ichimaru because she's going to try and stop him.
5) It's unknown if Ichimaru has been hollowfied like Tousen at the time of writing but I sure as hell hope not.
6) It's unknown which squad Isshin belonged to. The reason he isn't in the flashbacks is that Kubo Tite didn't want to reveal it of course. The canon reason will probably be that he just didn't appear (E.g. The captains of division 10 and 11 during that time were not identified.) or that he was already gone when those events took place. There's also the zero squad which has only been mentioned briefly.
7) The true extent of Aizen's involvement and plans hasn't been revealed.