I thought that even after Apollo 1, they were still using a all oxygen atmosphere, except at launch, because modifying Apollo for a safer mix was judged to be too far a redesign for the time and money they had.
The new atmosphere was 60% oxygen at 14.7 psi as opposed to 16 psi of pure oxygen at launch, dropping to 60% O2 at 5 psi during launch, with the nitrogen then being purged over the next day, giving 100% O2 at 5 psi. Suit atmosphere was 100% O2 at cabin pressure. In any case, at 5 psi, even 100% O2 isn't a significant fire risk.
60% O2 at 5 psi has the same partial pressure of oxygen as Earth's atmosphere at sea level (which is 14.7 psi with 20% O2).
A significant amount of flammable material was removed from the Block 2 CM interior, and a fair number of wiring faults were also fixed.
Since I remember that one of the changes needed was a thicker/stronger pressure vessel, I disagree with you saying that all the capsule accidents were correctable without major redesigns.
No stronger pressure vessel was needed that I've ever heard.