This computer is already associated with an Apple ID (when trying to download family purchases)

On my iMac I have five user accounts: 1 master family, father (me), wife, daughter and son. For iTunes, the family is used for family sharing and as is the organizer for family purchases.

My son's iCloud account is maintain through the Family account as he is under 13. I recently set up his user on the family computer and today started to build up his iTunes library so he can sync his iPod. iTunes was not logged into his iCloud account and had no music (it had not been used before). I entered his iCloud account and annoyingly it would not recognize the password I had recorded for him so I had to go through iForgot, Once in I went to family purchases to download a song that I had just purchased on the family account. It stated that this computer was not authorized to I authorized it using my son's iCloud account. When I went to download it stated:

This computer is already associated with an Apple ID.

You can download past purchases on this computer with just one Apple ID every 90 days. This computer can be used with a different Apple ID in 72 days.

What? It's a new set up....

I deauthorized the computer and reauthorized it using the family account, as the computer is registered to the family account. Why I tried to download again I did not get the error about but this time it said the computer was not authorized (even though I just did it). I appear to be stuck in a cycle of Apple ID association or authorization denial. I'm a little PO'd.

In the end I did it the old fashioned way and simply dragged in the music using family sharing (as this is switched on).

Beyond having to wait 72 days for the Apple ID to resolve itself is there any other options? Can I delete the entire user account of the computer and try again? I'd be willing to do that if I thought it would work...