MU doesn't work as a plugin, it needs to be part of core. However, all those others are either already plugins or will soon become plugins (some, like Micro-Blogging are covered by quite a few plugins). Buddy Press is pretty impressive, but I don't see it being any more major than Gravity Forms or Shopp or
wp-ecommerce. It's a very nice, rather large, pretty complex plugin. But it's still a plugin.
They can certainly contribute, either by getting accepted to actually commit to the plugin, or by offering patches. And yes, their patches could be turned down. However, there are only a handful of reasons to turn down a patch. The two major ones are that it's bad (insecure, etc), or that it doesn't take the project closer to it's ultimate goal. If it's the second, then just keep doing your own plugin, the core plugin isn't covering the same ground you are.
There will be multiple committers. I think the Health check plugin already has about 6 developers with commit access (I'm one of them).
I'm sorry, but I see a problem with that logic "since you misunderstood a word, other people might misunderstand a different word". While people might misunderstand it, the fact that I (not a member of the "other group") misunderstood a completely different word, has no bearing on that.