The database is going to be your best bet.
Way too expensive. I reckon my original plan might be the cheapest option yet, even if convoluted.
Also, you might want to consider using the cold wallet to protect the hot wallet, not to protect individual accounts.
I may be missing what you intended to say, but the cold wallet
is used to protect the hot wallet, which happens to have several accounts. Note that these accounts are not associated with external users; they all belong to me, and are just separate for administrative/accounting purposes.