Olaf, the beauty of open source development is that the tools (these days, at least) facilitate a kind of self-organization. Discussions around complex technical topics can be resolved with issues. Pull requests allow individual developers to submit changes, but that doesn't mean your fork gets overruled. With GitHub, you can put a project online as a maintainer who has the final say in terms of what gets pulled in. The point is it's all there for you, a model that has proven itself with tens of thousands of succesful projects at this point.
I don't doubt that Amiga development is complex, but it doesn't get any less complex without code being dissected and discussed in public, in a forum that is familiar to today's developers. If I were to say "yes" to your question about mentoring, it wouldn't matter if the code wasn't available anyway. The whole idea would be preposterous.
And of course things are a mess, but it's a mess because of both the practices around its development and the inevitable crust of time. Still, all of the toolchains can be improved. Perhaps some of them can be unified. The wonderful part is that, with an open source development model, someone can come in and improve some smart part in a spare moment, while another improves something else of interest. There is no need for a monolithic approach, though again, as the maintainer, you can reject anything you want during your code review.
Surely this makes more sense than the way things are done now. Surely a near-total industry shift to this model for everything except games and appallingly dangerous IoT devices tells us that this is something that works?
Personally I don't care who owns the so-called "rights" to AmigaOS, but if that bothers some folks, and/or makes it harder to collaborate on GitHub, then perhaps we shold simply look at collectively buying out these rights, once and for all? Why not pool our energies a bit and liberate them to the Internet, as should have been done 20+-odd years ago already.