Well, if you're a sedeplenist, then you must conclude that the New Rite is unquestionably valid since the doubtful Sacramental Rites simply cannot be promulgated by the Church's authority.
If, on the other hand, you believe that these men are infiltrators bent on destroying the Church, the mere fact that they concocted a brand new rite, and questions about their motivations for doing so, are prima facie evidence for positive doubt ... which is basically what Father Wathen was saying, except, since he's no sedevacantist, there's a problem with his position. They must have had some REASON for rewriting this Sacramental rite.
Whether or not you believe all the conclusions of Father Cekada's study, there's certainly enough there to constitute positive doubt. But now we're opposed by the genius, so I think we need to drop everything.