Stand Alone Apple watch

I have a similar question to supersupau.  My 12-year old son has been using an Apple Watch SE successfully on a T-Mobile Standalone plan for the last ~8 months.  The watch was set up/paired to my active iPhone (which has Verizon service).  However, my son has been blocked from installing and using certain apps on his watch that I would like for him to be able to use (e.g. Spotify, Strava) because the watch is set up under Apple's "Family Setup" option.

I have an old “inactive” iPhone without cellular service that my son could use to pair to his watch.

So here is my question:

• Is it possible to re-pair the watch with the inactive iPhone and still retain standalone cellular service on the watch?
• My son has tried to do this using the inactive iPhone, which is logged into his iCloud account.  At the pairing step in Apple's watch setup process, when he selects "Set up for Family Member", he gets a blocking message saying that a parent needs to set up the watch.
• Could we get past this step by (1) having me (a parent in our Apple Family) log into the inactive phone with my iCloud account, (2) completing the pairing process using the inactive phone, (3) logging out of my iCloud account on the inactive phone, and (4) having my son log in back to the inactive phone using his iCloud account?

Thoughts or success stories, anybody?