Hi Vivian - Welcome. I think I can answer a few of your questions. Let start with the last question and go backwards. I purchased a little hand-held saw/miter box combo in the dollhouse section of the craft store. If you get them there, do not get the saw with the interchangeable blades… the darn blade falls out constantly. You can also cut dowels with pruning shears or heavy duty utility nips. HOWEVER, a much easier way to go is to use straws instead of dowels - they are very strong and you can easily cut them with scissors (I’ve stacked a 3-tier cake with straws).
To measure, first make sure your finished tier is level. Then push a dowel or straw into the tier until you hit the cake board/plate (go straight, no angles), mark where you want to cut, then pull it out and cut it. Then use that one as a guide for all the others you’ll need for that tier (you’ll need to measure each tier separately). Be sure your tier is very level, and that you insert your supports by pushing them straight down (no angles, not even a slight one, or the weight from above cake can make the supports start to slide, then serious leaning or even collapse can occur).
Don’t know if we’ll be able to answer your buttercream/fondant question. This site is the discussion forum for Real Baking with Rose (aka Rose Levy Beranbaum, author of The Cake Bible). She’s our leader, and we, he devotees… you won’t find a lot of experience among these bakers when referring to buttercream that “dries” or prepackaged fondant. We’re pretty loyal to Rose’s scratch recipes, including fondant and Italian Meringue Buttercream (contains no shortening, marshmallows, or powdered sugar). I can tell you this, Rose’s fondant can be applied over her buttercream after the buttercream has had a chance to chill until it firms.
I’d really encourage you to check out a copy of the cake bible from your local library some time…. and stay tuned for Rose’s Heavenly Cakes which is due to be released this year.
Hope I was of some help to you. Don’t be a stranger.