We don't know for sure how many g forces they pulled as the Shuttle broke up. To put it in perspective, top air show pilots like Patty Wagstaff* pull as many as plus/minus 12 gs during a show without g suits. In some particularily violent maneuvers, the positive and negative g forces are alternating rapidly. Years of training and conditioning allow them to withstand those forces for a few seconds without blacking out.

Investigators are working to reconstruct the sequence of events as best they can from the available evidence. My supposition is that when the left wing let go, Columbia rolled rapidly to the left, exposing the cargo bay doors to aerodynamic and thermal loads they were never designed to withstand. The doors would've probably come apart very quickly and the Columbia would've began disentegrating. No one knows how long it took for the plasma to breach the crew compartment, but I think everyone hopes it was quick. It's even possible that the windshield might've given way (though this seems somewhat unlikely) because it would've been exposed to higher loads than intended. If that happened, death would've come very quickly for the crew.

* I've seen her airshow routine and frankly, it is astonishing. Some of the maneuvers she pulls are just incredible. She was a 3-time US aerobatic champion, and notice I didn't say "women's aerobatic chamption." In US aerobatics, men and women compete as equals. She won the national championship against all comers 3 times. To say that she's damned good is a severe understatement.