I can be almost 100 % sure about that the "ichimura" brothers weren't real living people, however I think that there was someone like kanichiro Yoshimura (Mibu gishi den) aka when the last sword is drawn", Kondou was assassinated not killed in action, he was killed by some choshu guy. but as you said hijikata was killed in action, hmm I thought that yamanami wasen't the real name of the vice-chief, did okita killed him? where did you read that?

EDIT: I didn't actually know that Japan had an emperor at the same time as a shogun, but he had no power back then..