Downgrading macOS from Tahoe to Sequoia: User accounts, Time Machine, firmware

After upgrading from Sequoia 15.6.1 to Tahoe 26.5.1 yesterday, I am forced to reverse course and downgrade because of audio distortion for which there seems to be no usable workaround or fix (I believe it is a known Tahoe bug related to coreaudiod).

I know that downgrading MacOS is fraught and risky, but I have no choice as I use my computer for composing music, and this distortion makes that effectively impossible. I don't need a step-by-step procedure here as there are plenty of references online. But I have an unusual system configuration that I am concerned about: files connected with my account are partly on the internal system disk, and partly on an external SSD drive. ALL of my personal files are on the external drive; the account-specific files on the internal disk are all configuration files placed there by (mostly third-party) applications.

1. Because MacOS is (at least under the hood) a Unix system and has UIDs/GIDs, how do I make sure that when I recreate the accounts on my system (of which there are several), they all have the same UIDs/GIDs as currently? Is it possible to specify those parameters when creating a new user immediately after an OS install?
2. There should be no need to restore my personal files from Time Machine, as I will (hopefully) not need to wipe the external drive to reinstall Sequoia. But I will need to restore everything that was placed on the internal disk. Is there a safe way in Time Machine to restore ONLY those files, as opposed to doing a COMPLETE restore of everything in the backup?
3. I have seen concerning reports from high-level contributors here that the upgrade to Tahoe involves an (apparently transparent) firmware upgrade. Does that upgrade need to be reversed, and if so, how? (That's an area where I would greatly appreciate a detailed procedure.)

Thanks in advance.