I'm in zone 5 (wisconsin) and lost 6 to the temp swings (and 4 more to chipmunks and/or mice chewing them up). I also have a few that budded out but only have a couple of leaves. I don't think wiltpruf would help- and inside storage could be worse than leaving them be. I kept them on the north side of my garage wall to minimize the temperature swings. Unfourantely this is part of bonsai and there isn't a whole lot that can be done.

I'm still watching a trident maple, a bloodgood maple, a paperbark maple, 3 of 5 E. beech and 2 chnese elms. On the elms I was told not to give up until about July 4th.
I have had a clematis come back in the middle of July so I will follow this advise.
5 bristle cone pines in one pot have made it thru fine but have lost a Japanese black pine and a dragons eye pine.
Paul