Hi Fishbone...
One thing you'll find about bonsai... when a tree gets bedded down for winter (especially where you are) you need to just let it be. Unless you have a species which can tolerate being indoors (I say tolerate, because there is no such thing as a tree which WANTS to grow in a house), you have to be pretty patient in the winter as your trees get their rest. And even having said all that, indoor tolerant species have their own issues to consider. ie misting... humidity trays.... sufficient lighting... balanced temps... air circulation... not being eaten by the household pet...
etc. lol
The most important thing you can do for your tree right now is protect the roots by putting it in the styro container you mentioned. I just get the ones from the grocery store, and cut a couple one inch holes in the bottom, then I fill it several inches deep with mulch. I put the tree in, and fill up around the pot until it's an inch or two above the soil of the tree. Going up the tree trunk is fine. It's very easy to remove the bark come spring. I take the tree and immerse it into a tub of water very gently, and all the bark mulch floats right off.
Because you have kept it inside from the sounds of it... putting it suddenedly outdoors may shock it... climatizing the tree by putting it into cooler and cooler spaces over the course of a week or so would be best. So a cool room, for a few days, followed by an unheated but enclosed space... followed by esposure to temps which will pull it into full dormancy, would be the safest thing to do.
As an aside.... I have friends who live in the midwest/central states who will use straw bales to create walls to act as insultation and wind breaks. Then they cover the tops with clear plastic held up by wood slats. It holds in the heat from the earth to a degree and provides the protection their trees need. Of course, you only have the one tree, so thats a lot of effort. But in the future, should you get more trees, that may be an option for you as well.
Good rule of thumb to understand... as long as you protect your roots well, and prevent wind damage, the tops CAN be exposed to freezing temps. The notion of not letting the tree be in temps below 40 degrees, is not completely accurate. It can handle it. But given where you live, the combination of weathers (high wind, and deep cold) is what makes it a tricky balance.
I've had my junipers, and all of my outdoor bonsai for that matter, exposed to temps in the 20s and even the teens. But they were out of the wind, in insulted styro boxes and mulched. My trees stay out all winter long.... protected..... but out. I've had no trees die from exposure to cold of that nature, though I do recognize that your winters are more extreme than mine.
Definently show some photos of the places you were thinking of sheltering it... that would make it easier to advise you on what would make your tree the happiest. Light is important, even if it's afternoon light. Though the growth on a juniper is pretty much nil in the winter, unlike deciduous trees who lose all their foliage and thereby their ability to photosynthesize, there is still a small degree of activity going on.
Easiest way to know if your juniper is happy is that the foliage remains tender or flexible. The new growth you'll get in the spring should be a very vibrant green, and very tender. How you'll manage its growth is by pinching the tips of that foliage between your thumbnail and your forfinger. If it is a dull green color and brittle/crumbly/snaps when you touch it, it's a good bet the tree has died.
Iron chelate can help with yellowing as was mentioned, but it's also good to make sure you are diagnosing if there is a deeper problem first. Knowing the condition of your roots is more important than trying to fix the cosmetic issue of iron deficiency. Because by the time you figure out that the yellowing foliage has not righted itself with the boost in iron, it'll probably be dead.
To that end you'd be well advised to pop that tree from it's pot in the spring, after freeze danger has passed. Look to make sure it isn't overly root bound, and address that problem if needed. It may need nothing, but it's always best to know whats going on under the soil. But there's a bunch of time between now and then to figure out how best to address your tree's needs in that area.
To answer your question about junipers... no... they do not all live by the same dictates. They vary by region and envoirnment as much as any species of plant does. Some confirs can live in the south others can not... etc. Some junipers live in very dry conditions, and don't like high humidity very well... and others thrive in it. Your juniper looks like a fairly standard procumbens nana which is normally offered by vendors. It would be unusual for them to offer other kinds as most uninitiated people recognize procumbens nana as iconic of bonsai, even if they have no idea what it is.
Well enough for now...
Sorry to have gone on so much.... but you've asked a lot of good questions... and that warrented a lengthy response.
Welcome to the family.
Kindest regards,
Victrinia