The rules are clear that any of the Household Account members can book flights using the total miles in the Household Account. The miles used are then taken proportionately from all Household members based on the individual account balance.

That is why you cannot book flights from a Household Account for anybody who is not in the Household Account (Amex 241 partner excepted)

You have to trust everybody in your Household Account not to steal your miles (Your Household Account member who has zero miles could log in and use everybody else's miles !)