Next Winter is more concerning than this Winter given your choice of method is successful this Winter.
http://www.wisconline.com/almanac/gardening/hardiness.html
Flush the soil well to remove any nutrients from recent feedings. Time release pellets are typical
and may pose a slight issue to make an attempt at dormancy.
Personally, talking about a single bonsai, given the circumstances, that unheated garage with a $15 seed germination heat mat underneath
would suffice. Unplug it when it warms in there. Also Winter dormancy requires little light if any. I've never had to do that, but on occaision
I too miss healing mine into the ground, and move them into my crawl space sometimes for weeks at a time to avoid the harsh drying winds in the teens to below zero.
Much like your garage, except my crawl space rarely goes below 35 F. and never any light. We had -18F here 2 yrs ago.
As mentioned, they still can dry and need routine checking for available moisture, which usually you'll find is much less frequent in Winter.
A protected cold frame bonsai needs more frequent waterings than in ground per experience. Snows and rains typically keep them hydrated,
but Winter droughts are common place. Wisconsin says a lot for demanding criteria for a plant. Mulching it heavy, healed into the ground in a protected
location outdoors should serve your new bonsai well. Some pots will crumble eventually. Cannot go wrong with mica pots healed in.
But that's later. Your current issue however does propose a challenge that you'll be able to help others with eventually.
It's next Winter I'm concerned with given your -25F. possibility and a possible 8 yr old bonsai from Florida.
Best of luck on the journey.