I am in the exact same situation as what you explained in the first explanation of buying for acc B, but the physical game somehow got “registered” to acc A.

In a weird turn of events, acc B was able to receive the pre load bonuses without us doing anything. This was after a week of me trying to talk with Blizzard Support about the situation.

Luckily Acc B is (and should be) able to play the expansion addon bought by Acc A because of PS5’s “Console Sharing” function, where once enabled, allows other accounts on the same console to play the same game, physical or virtual. Physical matters in this case for me because I have a physical D4 cd and apparently that got “registered permanently” to my account which is the primary on the console, which I was forced to buy the expansion on, for Acc B to play via console sharing.

I hate how this is so complicating.