Yes...you can also use flavored liquid coffee creamers. What ever the liquid measure is, I usually do half water and half creamer, or a little less than half water and a little more than half creamer so that the creamer flavor is not so diluted.
I have poured warm ganache over a butter cream iced cakes with no problem. The top was completely covered with the ganache with some dripping down the sides. Just make sure you refrigerate the cake after icing before using warm ganache. HTH
Yes, you can use Crisco. High ratio does produce a better butter cream though. If I am using high ratio(which I always do)and the recipe doesn't specify high ratio, I will use less high ratio than the amount of shortening the recipe calls for. Hope that makes sense.
I don't want to sound like a Debbie Downer, but I don't think you are quite ready to take on a wedding cake. Especially if you have never worked with fondant before. Also, square cakes are a bit more difficult to cover than round. Sorry to say but I think you may be in way over your head.
The sets come with graduating designs. In other words the bottom tier design is usually a full stencil design, the next tier a bit less and so on. It depends what you want your design to look like. If you want each tier the same then you don't need a set. You can also cover parts of the stencil if you don't want the whole design on the cake.
It is not necessary to cover a board in fondant. It depends on the look you are going for. A board can be covered with foil(comes in many colors)that is made for this purpose. Or any food grade durable paper. HTH