Having a four stall barn is probably a good idea for a multitude of reasons, but it all comes down to preference, budget, ect. Stall normally have some time of separation like bars, so if a horse doesn't get along and they're stalled next to each other it's not the end of the world... And if you have three horses you might find yourself with four somehow... That's how they work!! Lol! Always better to have to space and not worry then to stress adding a new stall. Or if you have friends over. If you have the means to have four, I would!
In the event of emergencies, it is very nice to have a trailer that has enough room for all the animals. Several things can happen that may require evacuation such as flooding, wildfires, and so on. Having enough space for everyone is probably the best way to ensure their safety, although again, not necessary.
Unfortunately I have almost no experience with climates like that, I live in the Southeast (SC and FL) and have not had much experience with freezing climates. Although I will say, temperature controlled rooms are wonderful (especially when it's hot, boy is that nice).
has some great posts regarding trimming and mapping. I would look at those posts and hit the link in their signature. Some great info there.
I've always been told hay and shaving storage should be in a shed that is covered and weather proofed, detached from the barn so in the event of a fire it has less chance of harming horses. A few barns I've been at have stored in barn, and nothing has happened, but I wouldn't chance it.
When I first started riding it took me forever to find my diagonal. If you still have to look at it, then so be it. What I did when trying to learn was go into it, and after a couple steps look and see if it was right. With time and practice, it gets easier to feel out. You'll get it eventually. It's okay to look for now. Your seat/balance/overall riding techniques will get better with time. Talk to your trainer/instructor about the problems your having with your lesson pony, they should be able to help you as they should know the horse and how to correct it.
I totally understand the re-rider questions. I took about an 8 year break in riding and I'm just getting back into it after purchasing my first horse. It can be overwhelming- but horseforum is a great place to get opinions and have some questions answered. Good luck to you!