Some considerations about Bishop Williamson's famous (and single) approval of Novus Ordo Mass attendance:
Although the other Resistance bishops would not go along with his statement, and although +W himself said in a later interview that he had gone too far, it has been noted that Archbishop Lefebvre was known to have given a similar approval in private.
That's funny, because +W's "approval" was also directed to a particular, emotional woman who OBVIOUSLY didn't "get it". She was not a Trad.
The consensus about this infamous exchange (which I was present at, BTW -- I witnessed the whole exchange in 4K) is that +W was imprudent for giving such one-off, personal advice to a woman in a public setting -- not just public, but uber-public, as it was being recorded for posting on the Internet. That imprudence is what caused so much trouble for the Bishop.
When should Trads go to the Novus Ordo? Never. It's dangerous to the Faith, and it's not permissible to place your soul in such danger. BUT -- what if you don't understand the danger? You're ignorant, you don't "get it"? You can't force someone's conscience to operate according to the lights and understanding that YOU have, but they don't.
This is a fundamental point that Trads have to understand. If you don't understand it, or don't get it, then you fall into the error of believing that all Novus Ordo attendees are malicious heretics, evil, mentally retarded, etc. NO! Some of them might be. But all you can be CERTAIN about is that each Novus Ordo Catholic hasn't figured IT out yet. The bad-willed ones will always be with us. But as for those of good will: Once they learn, once they understand, once the light bulb goes off, THEN they will join you in the lifeboats of Tradition and will never look back.
But they have to spiritually survive until that point. And in their ignorant mind, that involves attending the Novus Ordo. Maybe they're seeking more reverent Masses, etc. They slowly wake up to one truth at a time. It's a process. That's where their mind is at. Grace is working on them. Be patient.
We should pray that more and more Catholics enter this blessed track, this path to salvation.
Never forget that Novus Ordo Catholics have an excuse that no other false Church has. When N.O. Catholics go to Mass on Sunday (or Saturday night), their Church is clearly labeled as a "Catholic Church" which any research will tell you is the Church going back 2,000 years to Christ himself. NO OTHER HERETIC, NO SCHISMATIC can rely on that excuse.
When you ask a man on the street to direct you to the Catholic Church, he will point you to a Conciliar Church. Everyone just knows what is accepted as "The Catholic Church". N.O. Catholics RIGHTLY believe that is the safest place to be. The problem is: they aren't aware of the Crisis, and that the Modernists took over the visible Church structures in the 1960s, the details about Vatican II, etc. But their heart and their will VERY WELL MIGHT BE in the right place. We must therefore give each one of them the benefit of the doubt, until the contrary is proven by their attitudes, behavior, willful rejection of Catholic dogmas, etc.