This is also my first year keeping bees and I have been thinking about this quite a lot myself. I’ll write in what my plan is and the experts can weigh in to redirect us both. I started with a 5 frame nuc of Italians at the beginning of June, which was later than I wanted.
I think you should remove the flow super and add your second box immediately. You can feed them back the nectar from the flow frames now or save it for the winter. I only have two deep brood boxes which are mostly all drawn and full of nectar, pollen, and a lot of capped honey (or sugar syrup that I was feeding them earlier in the summer).
I’d add it now. If you have some drawn comb from somewhere that would be a big help. But if you don’t, I think feeding them syrup would help. Some seem (even in our climate) to have success with a single brood box but they are committed to feeding the bees fondant from mid winter until the nectar flow starts.
There should be no excluder between the brood boxes so the queen can move up to the upper box with the rest of the cluster.
I think you’ll have to if you want them to draw and fill the second box. If they don’t fill it up enough then it will just be more dead space for them. I fed mine until the second box was almost all drawn because the major nectar flow for my area ends mid-June (even though there may be significant nectar from suburban ornamentals and from aug-sep goldenrod and aster). The other question for the experts is if there are enough large brood cycles remaining to get enough bees to have to keep all the hive volume warm and guarded. From what I read on scientific beekeeping the sugar:water ratio isn’t that important.
Syrup for drawing foundation
I did what @Michael_Bush and others have recommended and used 5:3 sugar:water.
Michael Bush bee feeding
I’m going to wrap mine, have a quilt box under a ventilated roof. I figure it can’t hurt and should reduce the amount of condensation on the insides of the hive and the quilt box will hopefully reduce top condensation and dripping while still allowing vapor to escape through the ventilated roof. The location of my hive is well protected from the wind. Otherwise, consider some sort of wind break.
Maybe we’ll get lucky and have a very mild winter like last year. You guys probably are about the same temperature as us here in Ohio but I bet you get a lot more snow - which seems to be good (insulation) and bad (block hive entrance). Make sure your stand is high enough and maybe make a sort of awning to keep the snow away from the front of the hive and the entrance.