Setup Apple Watch for family Organizer using another member's device

Adult A is family organizer with an iPhone 8 Plus, ios 16.7.10. Adult B has an iPhone 14, IOS 17. The newer apple Watch SE 2 cannot be set up by ios 16 and iphone 8 not getting new ios versions. The watch has cellular, and its own line on Tmobile. It has previously been set up for Child D who also has an iPhone 8 by using Adult B's phone to perform the setup. There is an Adult C on the account as well. Child E is also on the account and has an Apple Watch set up using Adult B's phone. FWIW, all users also have wifi-only iPads.

Now Adult A (Family organizer) wants to take the watch Child E had and use it themselves (it has been reset and unpaired). To set the watch up, Adult A attempted via the iPhone 14, and after pairing and updating the watch and setting a PIN on the watch, Adult A is not listed as an option to be a user of the watch (Adult C and Child D are listed, Child E is listed but not eligible due to having a watch already). Adult A chooses to log in to Adult B’s phone, and successfully completes 2FA auth, but at the next screen, the process does not continue. (The “continue” button highlights, but then it jumps back to prompting for the account login). Repeating the login keeps returning to the login prompt (but without 2FA).

How to overcome this behavior and set up the watch on Adult A's account?