Most sedevcantists do not reject Pope Pius XII's holy week. This thinking seems to have started with those who formed the SSPV, and are either still there or have moved on from them, but they are not the majority of sedevacantists. They may make a lot noise, but they are not the majority of those who hold the position.
Archbishop Lefebvre certainly did not hold this position of rejecting the Pius XII liturgical laws, and if the resistance is about returning to the principles of Archbishop Lefebvre, which I agree with by the way, then why adopt a position so foreign to his thinking?
Please don't equate pre-1962 liturgy with sedes or the SSPV. At the first General Council of the SSPX in the early 1970's, Archbishop Lefebvre and the priests decided that SSPX priests should use the missal that priests in their country were then using, whether that was 1950, 1962 or 1965, etc., as long as it wasn't the Novus Ordo. (The Archbishop himself was using either 1965 or 1967 or a combination of both at that time - until 1974.) No one, including the Archbishop, had the authority to make laws about it. To be really legalistic, all priests should have kept to 1967 !
So until Easter 1984, for example, all traditional priests in England, whether SSPX or independent, used the 1950 missal. The Archbishop changed this for priests of the SSPX in 1983/4 to create uniformity in the Society because of the talks with Rome he was involved in at the time. "I couldn't have got anything earlier [than 1962] past Cardinal Ratzinger," he said. So it was a pragmatic decision, not doctrinal or legal, and the independent non-sede priests continued with the 1950 missal, which Father Ringrose uses now, I believe.