The Single Bullet Theory remains standing, Firm and unchallengeable.
A version of the SBT remains standing. It is firm only if you ignore the evidence against it. And it is certainly challengeable.
The current version is not the version that the WC counsel were pushing, as the many references in the evidence to "first shot" being the one that struck JFK attest. John McCloy believed that JBC was struck by the first shot and did not feel it right away. It was not until many years later that the second shot SBT gained acceptance. So much for a "firm" theory.
The evidence against the second shot SBT is abundant: over 20 witnesses said that JFK reacted to the first shot - not by smiling and waving for 3 seconds which is what the SBT requires. No one said he continued to smile and wave after the first shot. There is abundant evidence (from motorcade witnesses, photographers, witnesses along Elm) that the first shot was between z186-202. There is evidence that JFK was visible to Oswald in the SN by z195 and likely visible all the time he was passing under the oak tree. There is also abundant evidence that the shot pattern was 1.......2...3 so there could only have been one shot by z225 when the President is seen to be reacting. That in itself contradicts the second shot SBT.
As far as being challengeable, I am not sure what you mean. I challenge it. That does not mean that Oswald did not fire all three shots. He most certainly did.