That is a good and functional design.
For sure you should insulate the ceiling. That is where the worst of your frost will form, and in this climate you will get some frost even on the underside of a plywood-and-shingle roof (a metal or plastic roof would be a GIANT frost farm, if uninsulated).
Just end the insulation 6" or so back from the vent openings, so that the insulation is not blocking the openings or constricting them. That leetle bit of uninsulated roof edge will be totally inconsequential, and that way you keep the openings open.
BTW I do not know how many chickens you are thinking of putting in there, but if you are thinking in terms of stocking it at a 4 sq ft per chicken type density (which I really would not *recommend* in this climate, but, often people get away with it sorta-ok, although not always) then I would urge you to consider adding *additional* vents atop the walls, so that on nicer days you can open up significantly more ventilation than just the soffits there. This, plus droppings boards cleaned daily, would help minimize humidity and thus minimize frostbite.
Good luck, have fun,
Pat