Well the story of how MAME came to be really isn't much more than it evolving from Nicola's older Multipac project, there used to be a brief about this on the homepage, but it doesn't seem to be there on the current site.
As for how the project is run, it has varied over time. Nicola was in charge at first, then I was for a while, currently Aaron is. The project co-ordinator has some overall say of the direction of the project, but the majority of it is down to the individual developers so as you can imagine, getting a single balanced viewpoint that gives the correct overall picture could be difficult.
A good example of this is that personally I enjoy the challenge of figuring things out, and documenting them, I tried to push the project to be more open, and involve more people and to include all the knowledge we had, no matter how preliminary. I felt this was where the value of the project was. For this reason a lot of the things I've emulated seem like odd choices, unpopular games etc. but I'm usually still working on something emulation related.
I get the impression Nicola was most interested in the technical challenges, encryption schemes and the like, at that point in time everything was a technical challenge, today we tend to take for granted a lot of what was discovered / written back then.
Aaron seems to be pushing for more architectural changes, overhauling of the codebase, treating it more as a software project rather than an emulation project. I have a feeling this is because he was more motivated by emulating games he remembered, such as Atari classic era games, and the majority of those are now emulated and the remaining emulation challenges don't interest him as much. I'm not sure I agree entirely with this direction because I don't feel it adds real value, but others would argue it's invaluable and should have been done a long time ago.
but as I said, that's only a tiny fraction of the story, there are developers specializing in various areas, community projects built around MAME. I don't think a single developer can possibly give a complete insight into things, which might be why you're struggling to get any answers. I doubt this helps you cause much, but that's just how it is.