I take it you're suggesting the SBT is not essential to Oswald being the lone gunman? I have always had in the back of my mind that the SBT, while possible, is not critical to the LN narrative.
The mistake made by the WC was in assuming that JBC was struck by only one bullet. The significant speed of the bullet exiting JFK's neck and the absence of any impact damage in the car, meant that it must have struck JBC and caused all his wounds. Indeed, this is the reason Arlen Specter said he introduced the SBT.
Prior to the SBT, the working hypothesis of the FBI was that JFK was hit in the neck when the car was near the lamppost near the Thornton sign and JBC was hit by the second shot around frame z275 as represented in their models:


The third shot was at z313. This view was based on the visible movements of JBC beginning around z275, the 1.......2...3 shot pattern and the recollections of Secret Service agents (particularly George Hickey and William Greer).
The FBI appears to have assumed that the bullet through JFK's neck was spent and ended up in JBC's clothes without causing injury. When the Army ballistics experts said that it would have had plenty of energy after passing through JFK, there was a problem - but only if one assumes that JBC was hit by only one bullet.
I suggest that such an assumption was a mistake. The WC did not consider that the left thigh wound could have been caused by the neck bullet (at z190-200 when JFK was near the lamppost/Thornton sign) and then JBC was hit later by the second bullet that passed through his chest and struck the back of his radius.
At z275 the right forearm was turned so that the back of the right radius was facing the chest exit location. The evidence of fragmenting of the bullet in the radius is then explained without having to pretend that these two wounds were caused by CE399. That also fits with Tague's recollection that he was hit by a fragment on the second shot and with Greer's recollection that the second shot sounded like it hit something hard giving a concussion effect. His right ear was 12 inches from the dent in the windshield frame caused by the impact of a fragment.