After the Commission has directed a case for review, a person may move to intervene. A motion to intervene shall be filed within 30 days after the Commission's direction for review unless the Commission, for good cause shown, allows a later filing. Intervention before the Commission shall not be a matter of right but of the sound discretion of the Commission. The movant shall set forth:
(a) A legally protectible interest directly relating to the property or events that are the subject of the case on review;
(b) A showing that the disposition of the proceeding may impair or impede his ability to protect that interest;
(c) The reasons why the movant's interest is not adequately represented by parties already involved in the proceeding; and
(d) The reasons why the movant should be excused for failing to file for intervention before the Judge. A motion for intervention shall also show that the granting of the motion will not unduly delay the proceeding or prejudice any party and shall explain why the movant's participation as an amicus curiae would be inadequate. If the Commission permits intervention, the Commission's order shall specify the time within which the intervenor's brief and any response or reply may be filed. In denying a motion to intervene, the Commission may alternatively permit the movant to participate in the proceeding as amicus curiae.