Modernists like Rahner and Jurgens would have also loved nothing more than disseminating the idea that the Church had long admitted to contradictory teachings and substantial changes in doctrine, thereby paving the way for more.  I wouldn’t trust anything they said about Church teachings or history.

He was “ordained” via the Palmarians

People like this have no actual reasons.

Thuc's ordinations and consecrations at Palmar were certainly valid, and Thuc's consecrations of Guerard des Lauriers and Carmona/Zamora were also clearly valid.

Webster descends from those two lines, and the only quesition was that there was no record of Terrasson's having been conditionally ordained (having come from doubtful Gallican orders) ... and yet someone here once produced a copy of the ordination certificate for Terrasson that looked quite legitimate.  That is the only possible room for positive doubt in Webster's line.

Even if he had never left the Palmarians, that would have nothing to do with his validity ... unless one wishes to revive one of the earliest heresies condemned in the Church, that non-Catholics and/or heretics/schismatics cannot confect valid Sacraments.

Slupski conditionally ordained Webster before consecration, leaving no doubt. Also, Thuc did the ordinations and consecrations because Lefebvre vouched for Palmar. 

+Sulpski is dead but he came from +McKenna, I think. 

Okay then if Webster was consecrated and conditionally ordained by someone from +McKenna’s line then he is a valid Bishop.