I’m honestly not 100% certain what happens when a parent with a Scoutbook account takes YPT in terms of the creation of a new Scoutbook account. I suspect that the moment that the parent creates an account at my.scouting, a new Scoutbook account is created with the information listed, and the BSA ID generated is already attached to that “new” account (so can’t be added to the existing account manually). I think that the correct course of action in that case is to have the parent go into his or her account after completing YPT and use the Switch SSO Profile at:
My Dashboard → Administration → My Account → Switch SSO Profile
to map the my.scouting account to the parent’s account before submitting the application to the council. That way, at least, the BSA ID and YPT should be correctly associated with the parent’s existing Scoutbook account before the council gets to entering the new registration.
Honestly, in that situation, my next call would probably be to the registrar to see how they want the unit to indicate that the adult has an existing Scoutbook/my.scouting account. Most registrars are happy to work with folks, particularly when it reduces their workload. I’d let them know know what I had already tried (adding the BSA ID in the paper applications, making sure it’s mapped to the “existing” account via Switch SSO Profile,…) and ask what other steps would help streamline the process for them.
There’s not a whole lot that I think can be done on the unit side beyond that, and there’s no way for Scoutbook (I think) given the current structure to determine automatically that Josh’s dad Charley Hamilton is also Troop 750’s ASM Charley Hamilton if there are differences between the accounts.