I think a lot of this is either already well-addressed (like the participation model in Stream) or will become clear over time. You reference Stef's blog post, so I know you're keeping up on those things and don't need repeats. So, I'm not going to do a line-by-line reply. I do want to specifically address the parts where you ask about directly about me and suggest things for Fedora, though:
How about you, Matthew Miller? Can we go to you to get Stream SIG sponsorship? Are you even on the governance board for Stream? Do you have any personal investment in making us "very welcome" to form a BTRFS SIG?
I am not part of CentOS governance. As Rich says, the list of members is current.
However, I very much care about CentOS success as part of the whole Red Hat distribution family and a Fedora downstream, and because as I mentioned I consider myself part of the CentOS community. So, while I can't sponsor anything, I very much would like to see CentOS SIGs and Fedora teams working more closely together. I'm positive that the people working on BTRFS in Fedora would also like to see it in CentOS Stream. So I'm happy to do what I can to facilitate that.
Yes, we have a RHEL Insider Fast Ring called the Fedora Project. What would make perfect sense is to make the RHEL Insider Slow Ring called Stream fall under Fedora Project as well.
I've thought about this a lot! While I love things that grow Fedora, I think there's also benefit to a clear distinction. Since the Fedora Extras/Core merge, it's an underlying expectation that we have community participation and access across the board. There's no "only Red Hat decides this". With CentOS, on the other hand, the core expectation has _always_ been that Red Hat makes the fundamental engineering decisions for the OS itself while the CentOS community does things _around_ that. It's a different kind of project with a different kind of participation.
CentOS Stream doesn't change that expectation for the CentOS Project; it's just a better version of the model: discussion and pull requests welcome, rather than "well, you could file a bug with Red Hat and hope someone notices".