The kabaneri need to drink blood because their kabane heart requires it, it seems to me. Hard to say if they need to drink no matter what they do or only if they really use their kabaneri powers, but it would make sense using more power means they get thirsty sooner. If they are still half human, they should be able to produce their own blood like human do, but it hasn't been verified yet. It also might not be enough.

I had a big problem with Mumei listening so keenly to someone she obviously despises. But that's how I always feel when in typical shounen the hero is always ready to listen to the words of the villain and even believe it all, no exceptions. This time Mumei took advice from a failed person so much to heart heart that she threw all that she had achieved so far away and proceeded to do dangerous and stupid things. Right. It also made no sense whatsoever, no matter how you look at it. No idea what kind of a scumbag this brother of hers is, but would he possibly care less about the train full of people than randomly killing random kabane in a lost town, not that he would ever hear of those slain kabane, but if that train ever reaches a safe town, he would hear about it. It's like Mumei totally lost her thinking faculties when the pirate talked to her.

Another annoying thing is Ikoma's attention span of a five years old. The moment something new catches his eye, he totally forgets what he was currently doing and for what purpose and runs for the new toy. It's like that every fricking single time.