It's always difficult to diagnose these types of problems, but I think your problem was a combination of 1) too much yeast, 2) the effects of freezing on the dough when you chose to freeze it, and 3) too little sugar to sustain the dough over a 3-day period. None of this was by plan, but I believe the combination of the above factors may have led to the problems you experienced. Let me see if I can walk you through this.
Usually the amount of yeast you used shouldn't have been a problem. However, when you decided to freeze the third dough ball, it had already risen enormously--because of the large amount of yeast. Freezing at that time would not have been the optimum time to do so. Usually the best time to freeze a dough and do the least amount of damage to it is shortly after making it. In your case, by the time you decided to freeze the dough ball, it had expanded significantly. It, in effect, became like a hugh insulator filled with gas pockets. Freezing the dough at that time can cause significant damage to the yeast in the dough as the moisture in the dough expands through freezing and ruptures the cell walls. Also, the damage to the yeast can cause fluids, such as glutathione, to leach from the cell walls and result in softening of the dough once it has thawed. In fact, some professionals often intentionally add glutathione (dead yeast cells) to their doughs to combat elasticity problems.
The high amount of yeast you used also most likely caused the dough to expand too quickly and foreshorten the optimum fermentation time (by consuming all of the available sugar too fast) and, consequently, the useful life of the dough. The reason I asked you whether you temperarature adjusted the water is because if the water was too warm the dough would have risen even faster and also foreshortened the optimum fermentation time. I don't think water temperature was a causative factor in your case because the finished dough temperature was about where you would have wanted it for a Lehmann style dough.
As for thawing the dough, the advice usually given is to thaw the dough in the cooler/refrigerator for about 12-16 hours and to use the dough the next day. This is the advice given for a dough that has been properly frozen to begin with. In your case, I don't know that that would have really mattered since I suspect that your dough was already long in the tooth fermentation-wise. If you had used more sugar in the recipe to start with, the dough might have had more life left in it. You didn't indicate in your posts, but was the dough softer than usual or different in any other material respects from what you usually experience as you shape your doughs? A really soft or slack dough is often a symptom of overfermentation or damage to a frozen dough (because of the leaching of glutathione as mentioned above). Also, did you detect any change in the degree of coloration of the crust from what you usually experience? Usually, the crust will be much lighter for a slack, overfermented dough. In your case it might not have been noticeable since your very high grill temperatures may have masked that effect.
I would be very surprised if your problems were due to the biga. Have you ever tried using the biga alone without additional commercial yeast? That would tell us how potent your biga is. If it's weak, then it most likely wouldn't have been a causative factor in the problems you experienced and, more than likely, it was overwhelmed by the commercial yeast, especially in the quantity you used.