Can you use scratch grains as a winter supplement?
I use whole corn only as a supplement I know it is high in protein and does the job. I can not really comment on scratch grain as all scratch grains are not created equal.
Is a roof vent and vent holes under the overhang enough ventilation, you think?
It may be all you need time will tell if it is adequate you are definitely on the right track.
Monitor you coop daily and if you have ammonia problems and moisture problems you may have to create more ventilation. It is all about balance number of chickens vers amount of ventilation. In some coops the deep litter method adds heat to the coop (the deep litter in my coop freezes as hard as a brick until late April or May).
I was planning on covering all my wire with thick plastic for this winter.
Sounds like a good plan you can always create a few flaps in the plastic if you are having ventilation issues.