I remember chatting with you on another thread. I can't remember details but probably about your coop. Thanks for the background info, that helps. Your goals and situation are different from a lot of us, and you certainly can get different opinions.
I'll give you a link to an article by someone that deals with colder temperatures than you. This is more for management techniques than actually building and outfitting the coop.
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
Do you agree with the hatchery that these breeds are cold-hearty?
Your biggest danger is frostbite, not that they will freeze to death. You avoid frostbite with good ventilation but avoiding a breeze blowing on them directly. Moisture in the coop leads to frostbite so you need to get the moist air out. You also want to avoid a breeze blowing directly on them, party because of wind chill but also because their down traps air in tiny pockets which provides insulation. If a breeze ruffles their feathers it can let those air pockets escape and they can get cold. It's not about keeping the coop warm for them, it's providing conditions where they can take care of themselves. If you try to pack the coop tight enough for body heat to make a difference you are very likely to have behavioral problems from them being so close together.
Chickens with large combs and wattles are more susceptible to frostbite than chickens with smaller combs and wattles. All your breeds are single combed and can be more at risk than birds with smaller combs like Wyandottes, Chanteclers, Buckeyes, or Dominique to name a few, but plenty of people keep even Leghorns that have huge combs in your climate. Other than the combs your breeds are quite cold hardy. Just watch out for frostbite
I learned that they'd need 2-3 sqft each in the coop, and 10 sqft in the run.
You may find that those numbers work for you or you may have issues. Every situation is unique and yes, this is another of those where everyone has an opinion. If you follow the link in my signature below you can see what some of mine are about room requirements.
I heard that 4-5 hens should give us a dozen or so eggs / week. Is this correct with the breeds I have raised?
As others mentioned, it will vary by season and by age. Each hen is an individual. Breeds have tendencies but you have to have enough for averages to mean much. Go by your experiences rather than rely on magical mystical numbers you read on the internet. Still, with four hens of those breeds in peak production don't be surprised to push two dozen eggs a week. In the off season it can be a lot less.
Some pullets (not all) tend to lay throughout their first winter whether you provide extra lights or not, but expect production to drop some. Lights may help keep production up. Their second fall they should all molt and stop laying until they regrow the feathers. Some might start laying in the dead of winter when they regrow feather or they may wait until the days get longer in the spring whether you provide extra lights or not.
Providing lights is a two edge sword. It can certainly help production during the winter. But after a hen lays continuously for a length of time, maybe a year or maybe a year and a half, production drops and the quality of the eggs can suffer. They need to take a break, molt, and rejuvenate their bodies for another round of good egg production.
My Initial thought is to keep 1 of each breed (BO, BR, NH and Golden Buff) to see which are the best layers. Of the breeds above - which should I keep as layers?
The ones that your experience tells you are your best producers in your conditions and with the individual hens you have. We all have our favorites but that's more opinion than fact. Also it is based on our experiences which will be different to yours.
At what age should I harvest dual purpose chickens, and at what age should I harvest surplus cockerels?
As always different ways to go about it. As they get older a hen lays less. Typically a hen lays really well her first laying cycle. After she has her first adult molt, usually the fall when she is maybe a year and a half old, her second laying season is also outstanding. After her second adult molt she can still have a good laying season but production starts to noticeably drop off. This is typical of a large enough flock for averages to mean much. Each hen is an individual, some do a better than this, some do worse. I tend to butcher mine after their second laying season when they start to molt, say around 3-1/2 years old. Others use different systems. That's for the hens. These hens are great for Coq au Vin, Chicken and dumplings, soup, stew, or broth among other things.
There are a lot of opinions on when to butcher cockerels. A lot depends on how you cook them. As they age they develop more flavor and texture. When quite young you can grill or fry them but they wont have much meat. Wait for them to grow some and they can still be roasted. A little older and they may need to be baked or cooked some other way. I personally target 23 weeks as they have hit the end of their youthful growth spurt. I prefer baking them, they are certainly too old to fry or grill. But others do it differently for their own reasons.
You need to work out your own system. From what I understand of your goals (which are subject to change as you gain experience) I'll toss out an idea for you to consider.
Say you want to over-winter 5 hens and you are buying all your chicks. Every spring buy 5 or 6 female chicks. Raise them until they all start laying and you can select which chicks you want to keep over the winter after you have butchered your oldest hens. At the same time order a bunch of cockerels to be grown out for meat. Don't order straight run, order the chicks you want. There is no guarantee how many or either sex you will get with straight run. I once got 7 pullets out of 7 straight run chicks. If I were counting on getting a male to be my rooster I would have been out of luck. If you count on getting pullets out of straight run you might be unlucky.
Good luck however you choose. You have so many options and ways to go about it.
I have to be careful because I can quickly get down to 6, and then I'm in the hospital with a dreaded blood transfusion. I think the juice is my answer for the future. 
