Does Comcast actually allow customers who own their own modems full management of the modem firmware? As far as I have been aware since my time at Adelphia 20-odd years ago, that has never been allowed by provider; all users of a given model had the same firmware enforced, customer owned or leased didn't matter.

No and I am not aware of any DOCSIS network operator that does permit that. But we are very responsive to firmware updates from the OEMs and try to quickly test & deploy those. See for example: https://kb.netgear.com/000036375/What-s-the-latest-firmware-version-of-my-NE... Thx Jason