In any proceeding involving § 458.26, a trusteeship established by a labor organization in conformity with the procedural requirements of its constitution and bylaws and authorized or ratified after a fair hearing either before the executive board or before such other body as may be provided in accordance with its constitution and bylaws shall be presumed valid for a period of 18 months from the date of its establishment and shall not be subject to attack during such period except upon clear and convincing proof that the trusteeship was not established or maintained in good faith for purposes allowable under § 458.26. After the expiration of 18 months the trusteeship shall be presumed invalid in any such proceeding, unless the labor organization shall show by clear and convincing proof that the continuation of the trusteeship is necessary for a purpose allowable under § 458.26.