You didn't post your actual state so some ideas. 1. Our barn puts on sheets when the temps are a continuous 40F and start blanketing, most of us in heavy weight blankets, at 20F. All of our horses are stalled at night with doors closed so it is actually 10 degrees warmer inside the barn even in fairly high winds. The early sheeting for us is more a matter of convenience to keep coats short and easier to clean/dry as many of us show all winter. Some of the horses do have naturally very short "winter" coats so for them blanketing is important even with the increased hay rations the horses get in the winter.
2. If you allow your horses to develop a winter coat, maybe not sheet or blanket before say 30, it allows you to possibly use a lighter weight blanket...medium vice heavy. What is important is that the horse stays dry. If you are turning out the blanket, no matter the weight, should be waterproof.
3. At the lowest temps you are indicating, and thinking a windy night outside, I would use a heavyweight no matter the coat length. Wind takes away body heat. Now, others will say allow the winter coat to develop and let nature take its course, leaving the horse unblanketed, unless the horse is elderly, ill or thin and is having difficulties.
4. As for indoor riding and then turnout, again, depends on the horse. Making sure the horse is 100% dry is imperative. If they are used to being turned out and again, the wind is "normal" for your usual unblanketed status, I wouldn't worry about it. If you are worried about sweating, do a hunter style clip or chest line clip but then you would want to have at least a lightweight blanket.
5. As for swap times, it depends. There really isn't one set of standards that says, when it hits this temp change to this weight blanket or else. Let the horse tell you. If you blanket too heavy too soon you'll find the horse sweats underneath...that would obviously tell you to switch to a lighter blanket or remove it entirely. Understand that as you sheet and blanket, you inhibit the coat length as the horse's body doesn't register such a difference in the temp change.