Re: Winter is killing my Dwarf Cavendish, please help!
As rule of thumb, I don't touch/separe/prune/repot/anything on any banana during winter. They already have hard time getting accustomed to the indoor environment, with constant (low) temperatures and no temperature swing between night and day, lack of light and so on. Just treat the pup as any other pup at this point.
When it comes about the mother plant, i guess that your best chance are reducing watering (to discourage any rot may be forming) and let it stay in the most stable environment you can provide it. I wouldn't repot at all, unless you are desperate. I don't know how long your plant must have been exposed to 20F but yellowing isn't sign of cold damage: cold damage makes the leave with a "cooked" look. So i guess root rot may be the cause, which makes the suggestion to avoid watering even more needed.