5 or 7 seem best. I agree with you that 9 is too large initially. If it’s 5, what is the quorum (3, 4, or 5) to have a meaningful decision?
I agree with you that a subsidiary can be viewed as a different employer (LinkedIn and Microsoft).
As for money exchange and bylaws amendment, I believe that it depends how the handling of money would occur. I could see the PSF earmarking a lump sum each year for CPython where the specific decision making on the funds, as allowed by 501c3, is based on the core team’s views. This could be a group as suggested in this PEP a) acting as a workgroup, b) having fiscal sponsorship with the PSF, or c) requesting funds as needed to be voted on by the PSF board. I think a) and b) would lend for smoother operations.