It is good to know which stars the captain is using, to guide his ship by.
When trying to understand O Sensei I think this is true, but when I think of training as a personal thing this doesn't ring quite as true. O Sensei would be captain of his own ship, not mine. As such, it might be useful to know all the particular aspects that compelled him, but as it applies to my own ship, it really only comes down to whatever I choose to apply (i.e. whatever seems useful to me and my goals at the present). I think this is what Mary is pointing to.
Regardless of the facts or the fiction, each of us still has to internalize what we're learning and that, as I see it, is a purely individual endeavor. History is never 100% concrete. I learned this from my classics prof. first year in college. The value of any history, as I see it, doesn't usually come from whether or not it's true. It usually comes from how we interpret it and apply it to our own way.
Regarding O Sensei living for the sake of others I think it's important to consider the idea that by helping others we help ourselves and the overall state of things. I know there are philisophical discussions about whether selflessness truly exists or not, but I believe O Sensei believed in helping make better people more for the sake of something greater than those people or himself. That's not to say it wasnt also for their respective sakes as well, but I'm betting there was a hierarchy of importance to him that started with the aspects most fundemental to existence itself.