When you plastic sealed your windows did you use any felt roll or clay coil type of weatherstripping sealing too?
We use the felt weatherstripping which comes in a large roll, either with or w/o adhesive backing. We open the window & put it on the bottom of the window then close tight (it will be somewhat hard to shut). Then we place it between the spot where the single hung windows meet.We add the coiled clay type insulation to the spot where the windows meet in the middle, on everywhere the window meets the frame. Add the clay to the outer edges of the window too. Then plastic seal.
The clay rope type weatherstrip insulation is easily removable. If you do not get the glue backing felt type insulation you'll need to tack it down which you may not want to do in regards to the landlord issues. I've never used it for this but Goop may work to remove any glue residue.
Also you might want to add some Velcro to the curtain & to the window frame or wall in order to close the curtains tighter.
Like shortbun, I also pile throw pillows on the sofa back to keep the drafts off us. I got a cheaper twin electric blanket to use in the family room when it gets bitter.
We live in a 225+ yr old tavern in central New Hampshire. Tonight it is 5 below w/winds from 20 -30 mph, brrr!
Jean