1) I like your idea of shifting the whole 1.5 tree to 1.6. We can use Apache to map URLs from standard URLs without version numbers to the latest. The other path would be to have clearly marked sections in the same page that says "These options were added in 1.6" and so on.
2) I definitely agree we will have to have some sort of authentication for editors. Nothing too restrictive - just something so we can track people in case of problems. I'm unsure of how this is done in MediaWiki but I know it's possible.
3) I don't think it'll be that bad at all. Those buttons would actually be on course/view which makes good sense to me. Help for exceptions like course/mod.php can easily be made an exceptional case in the code to take us to mod/forum/mod and mod/quiz/mod (for example). And for different views, it won't hurt to have the common view described at the top, with addendums to describe additional features for teachers and admins if there are any on that page .... it will help people understand better how it all works together.
On top of this page-by-page help there will also need to be some sort of longer manuals, for sure, but at least they can link to any of these other smaller pages with a strong confidence in the URLs.
I've just upgraded the MediaWiki to their very latest version - 1.5 alpha. I'll try and make a new "Documentation" theme soon that includes the special links in the header, to make it easier to get started in Moodle 1.5.