At no time after the issuance of the complaint shall the Administrator, the members of the Environmental Appeals Board, the Regional Administrator, the Presiding Officer or any other person who is likely to advise these officials on any decision in the proceeding, discuss ex parte the merits of the proceeding with any interested person outside the Agency, with any Agency staff member who performs a prosecutorial or investigative function in such proceeding or a factually related proceeding, or with any representative of such person. Any ex parte memorandum or other communication addressed to the Administrator, the Regional Administrator, the Environmental Appeals Board, or the Presiding Officer during the pendency of the proceeding and relating to the merits thereof, by or on behalf of any party shall be regarded as argument made in the proceeding and shall be served upon all other parties. The other parties shall be given an opportunity to reply to such memorandum or communication. The requirements of this section shall not apply to any person who has formally recused himself from all adjudicatory functions in a proceeding, or who issues final orders only pursuant to §22.18(b)(3).