I would be wary about saying that the Holy See has no jurisdiction over Festing or the Knights.  Par. 1, Article I, Title I of the Constitutional Charter of the Order of Malta very clearly says that the Order "is a lay religious Order".  Par. 1, Article IX,  Title II of the Charter also states that the Knights and Chaplains belonging to the First Class - and Festing is one of them - "are religious for all purposes of Canon Law ..."  Now, since Festing is a "religious for all purposes of Canon Law," he may well be subject to Canon 590, No. 2, which says that individual members of institutes of consecrated life are "also bound to obey the Supreme Pontiff as their highest superior by reason of the sacred bond of obedience."

Considering that Festing and his cohorts have all but slammed the door on the investigative commission created by the Pope, his "highest superior", I seriously doubt he will come out of this fiasco unscathed.