I don't use plastic. Mostly for the reason you mentioned. You don't know how long it has been stored and how much wax remains on or was on the plastic. To remedy that, a few folks I know, place burr comb in a crock pot, melt it and paint it on the frames with a brush. They say this helps with getting the comb drawn.
As far as drawing the deep, I have had no luck with it after the flow stops. Also when I have feed they just start backfilling and then you run the risk of crowding the queen. All I end up with is new foundation with chewed comb, of course since you are running plastic you may have better luck. You can try to place frames in between drawn frames and try to force them to draw them this way. When they draw it, if it is all honey packed, pull it place it away from the hive and have the bees rob the honey to leave drawn comb. This is the way a queen producer I know gets late comb drawn. He also states after the flow it is hard to get them to draw, but if you keep an artificial flow going at the same time the natural flow stops they will continue to draw. I was just talking to an old timer beek in my area a few days ago and asked him the same questions about getting comb drawn. He said he has some luck feeding 1.5 (s) to 1(w) for feed vs other ratio's. Just his opinion.
Keep a close watch on the SHB in those splits--- I have learned the hard way :'(
I get no late flow here in the edge of the Piedmont, maybe you will fair better down your way. Keep me posted on the outcome. Consider updating your profile from hopelessly lost :)