Because Itachi is a lot more important story wise and gets new narratives in part 2(especially in relation to him sealing Orochimaru with the totsuka blade, Izanami, stopping Obito from invading Konoha, being revealed to be a double agent) etc etc. You're essentially arguing that Kisame was retconned because, as an Akatsuki member, we see him hunt a tailed beast. Like, that's his job bro.
The statement is obviously iffy, but I don't think you can get around that it clearly implies that Kisame did not believe he would win. As long as it is pretty much directly told to the reader, there's not really much anyone can do about that. Mangaka usually don't make obvious PL statements like that.
Gai is high Kage level with the gates(correct me if I'm wrong and you mean base gai) and we know how big of a powerup that is from part 1 itself. We just didn't see him go all out in part 1. And yes, it's dumb that he didn't use his powers during Konoha crush, but it was established that he and Lee are unconventional like that