Anni, the poor thing sure has gone through some miserable times hasn't it.
About the soil. Fresh potting soil or mixes, have in them fertilizer that can feed a plant for a limited time.
Depending on the size of the plant and its feeding habits, the fresh potting soil should do sufficiently until the plant begins to grow when sunlight returns to intensity in February. Until then, just try to keep it alive.

Dracaena is one plant that is badly affected by fluoride in water. If you take water from a tap on a municipal system, you'd be advised to let it sit --at least overnight--or use some bottled water or water melted from snow and allowed to gain room temperature.

Corn plant also can take bright light...but isn't fussy about that; takes infrequent watering---isn't fussy about that; average house temps...not fussy there...but you are wise to direct heat vents away from it.
Humdity too doesn't have to be addressed..it can take high or low moisture.

The time interval that it suffered dryness I'd say was the thing to be concerned about. Add to that the low temperatures in the garage it may have dried somewhat--especially the roots.

To that you may think to soak your plant well. When you water, water well to drainage and let the water in the saucer sit there for 10 - 15 minutes...give the plant a time to decide whether it can use any of it before you dump it.
If the plant perks up in short order, you can rely on it furthering its health. If it doesn't I would be inclined to consider repotting with fresh new soil...no fertilizer needed...and another full watering to drainage.

Sun intensity at this time is pretty low so I wouldn't worry about giving it as much sun as you can...at least until it shows it is improving.
Whether actually the fluoride has done something to the plant is the big question.

Bumps on stems are nodes....cut back just above a node and be sure to make the cut cleanly.
The section of the dracaena that is cut off can be put into water until it roots up.....roots 1" long can be potted up..may take up to 6 weeks. Along the stem, swelling will occur and new growth will occur at the tip.
Often the portion you cut off, can be cut into sections of 4", laid on their side in soil half buried in the soil and it might root up that way.

The brown spots are most likely delayed reaction to the cold; there's not a lot you can really do about that except cut away the dead parts as they show up.

I don't know that the vent is a major concern, but everything else being equal, the plant is probably better off with the vent closed than it would be with it open.

The cut-off stalks would probably be having some yellow leaves even if everything had been ideal for months -- it just takes a while for plants to adjust to growing in water, and they'll usually throw some leaves while that's happening. I'm fairly certain that those stalks will do everything they can to root, but they've already been through a lot and this isn't the ideal time of year to try rooting a stalk, so you might be asking more of them than they can give. If they were my cuttings, I would probably leave them in water until one of two things happened: 1) they developed a pretty substantial root system and could be potted up in soil, or 2) the stem rots. Either way, you could be looking at a couple months' wait. Change the water every few days with tepid water (I think tap water is probably fine, but reasonable people can disagree about fluoride/Dracaena issues: make your own calls there.) and set the cuttings somewhere bright but not in any direct sun. Whatever will be, will be.

It's possible that the soil is/was too heavy; it might also just be that the plant wasn't taking up water at its normal rate, because of the stress, and the soil consequently stayed wet. Corn plants (Dracaena fragrans) are not good plants to leave in a heavy, wet soil (they'll rot, though they're less touchy than some Dracaenas about this), so depending on how things play out, you may want to change the soil eventually. For right now, though, I would let the plant catch its breath. A SE exposure isn't what's usually recommended (it's a little on the bright side), but corn plants are very adaptable, so I'm not terribly worried on that count.

I have since double checked the soil I used to replace what was lost. It is Miracle Grow Garden Organic Choice, all natural organic soil. I tried not to disturb the roots, merely straightened the plant up & added the new soil to the top. I think the water I added was from a well, not city water, but was not room temp. The new soil I added is now dry. I moved it a little further from the vent & the light is not strong as I have a covered deck over the French door.I've begun trimming off the dead/dying leaves, that are all yellow or mostly brown/dried spots. Waiting on nature & God. Thanks for your help!!