Is it necessary to inform the brokers of the amount the seller has allocated to the intermediary as commission, and is it permissible for him to divide this amount with one of the brokers to the exclusion of the others?

Your relationship with the brokers is a permissible brokerage partnership. All the brokers are partners in the amount the seller gave you, and they are entitled to their share of it. You do not have the right to claim the entire amount without them, or to give some of them and deprive others. If you do not agree on a specific method for dividing the amount, then it shall be divided equally among you.