not got a lot of time (always the problem, no? ) so I'll just answer main questions for now.
id agree with most of your points, but yes, most refer to larger open source projects, when the coordination is essential, and also the 'maintenance' of documentation etc is important, and most importantly there are volunteers willing to do it.
master - is basically, current release plus essential bug fixes.. i.e. if we were to do a 'maintenance release' this is where it would come from
experimental - is where we (johannes and I) are working for the next release, assuming we dont find anything that goes horribly wrong, hence why experimental rather than dev
you'll also see a new chibios branch with johannes is doing, this once stable will be merged to experimental
.. so if its just small bug fixes, master would be a good place IF you know it doesn't jeopardise the branch with bugs.
dev environment, run platform_x/build.sh, this will get most things required.
for java dev you should use netbeans, since this is used for the dialogs etc, but other than that you can just run ant
docs , user guide is built dynamically from this forum... you can see this in build.xml, apart from that yeah, the docs are a bit dated
roadmap, its johannes project so if you want to do something contact him, he can let you know if its in line with what he wants to do, and also if other work is being done/considered that may clash with it. this is what i do.
yeah, I know , seem loose (hell, its not how i run commercial projects ) , but its pretty common for small projects, where 'one man' has the main vision and drive, and others tend to dip in and out occasionally.
so, if you plan to do some dev, contact johannes with your ideas, then of course once you have spoken to him, you could update the documentation if you feel the desire