OFBSLF
If the tanks get punctured, could that be enough to cause the tanks themselves to fail structurally?
The answer is very probalbly yes as they are thin walled and are designed to withstand structural loads only when pressurized (just look at a plastic soda bottle just before first opening, and just after).
This being said, I would support Grainger when he says :
Once this had happened it's largely irrelevant what the Oxygen and the Hydrogen did
Nevertheless, the answer is quite straightforward : if they were allowed to meet, with the slightest source of energy available (may I remember that we have some rocket engines running at full throttle just below), there is only one answer : BOOM. Period.
Even if the tank is not punctured, if the booster rotates, they are doomed, because the controls will probably not be able to prevent the shuttle to veer off trajectory, and I'm quite sure the maximum structurally admissible angle of attack is quite low. Anyone got figures ?