In my mind, it takes more energy to make a second bubble to contain a huge explosion in an area much smaller than the warehouse, than it does to make a second bubble to keep only a small area inside shielded. HG could only draw out so much energy, the rest of the shield was supposed to keep the explosion from affecting the outside world. So it was either the Warehouse or Univille. Both couldn't survive.
That's how I make sense out of that scene....and as JMS would say, "It's my story and I'm sticking to it."
But the smaller shield was still exposed to the force of the explosion.. And it was right next to the bomb so it would have felt the full force of it..
It seems more than likely they're going to use the watch as a reset button.. but since blowing up the Warehouse clearly wasn't necessary, what are they really trying to undo?
If the shield held the bomb it would have needed to contain the whole explosion in one small space. All that energy compressed and held in such a tiny space with nowhere/nothing to expand into would likely take far more power to contain than it would to withstand/deflect the sudden and short (yet powerful) shockwave, heat, debris etc etc.