Fructose is one of the sugars in corn syrup. The problem is that 112 C is above the caramelization temperature of fructose, which is 110 C (230 F); this is uniquely low among the various common sugar molecules, most of which begin to caramelize around 160 C (320 F).
The toffee flavor that you are getting is due to the caramelization components.
However, without seeing your exact recipe it is hard to know what to change. I would suggest using table sugar with its much higher caramelization temperature instead of corn syrup would be the most appropriate solution.
It should dissolve the dairy, so you should not need to make a syrup.
However, sucrose (table sugar) has different crystallization properties than fructose and corn syrup, so after you get the main part of the candy mixture up to temperature and then cooled some, you might wish to add a tablespoon or two of corn syrup to help reduce the likelihood of graininess.
You could also try reducing the temperature to say 108 C, but this may affect the final ratio of water to sugar as less water will have been evaporated off, which would change the crystallization pattern in the confection, and thus is texture. It could end up quite sticky or even gooey.
It will probably be easier to search for a recipe for this confection that is already tuned to do what you want, as modifying candy recipes is very tricky; you must get the science and technique just right.
Note: fudge by its very nature is a a solid suspension of very tiny sugar crystals embedded in a dairy/syrup phase. The cooling and beating in traditional fudge recipes are to control the growth of the sugar crystals, so that they are numerous and very, very tiny (which gives the smooth silky texture) as opposed to larger and fewer (which gives a grainy texture).
As fructose and corn syrup tend to resist crystallization, it is odd to have a fudge like recipe with corn syrup as its main sugar component.
The issue of fondant versus buttercream is one of convenience. Fondant is (relatively) sturdy, pretty, and can be held for long periods. In a wedding cake, which might be prepared and decorated over several days these are advantages. True buttercream does not hold very well (it is at its peak only for a day or so) and is much softer, and it requires refrigeration, so it is logistically more challenging.
Other than that, the fondant versus buttercream issue is not really relevent to the structure of your tiered cake.
Typically, each layer would be on a cake round (or in your case, square), supported by dowels through the layer below. These dowels can be any food-safe material such as plastic or wood. The cake rounds (typically coated stiff cardboard) are essential to the structure, as they provide direct support to each layer, while the dowels serve as structure to hold the round up.
The Wilton site has a pretty good illustration of the method.
Best Answer
Fudge icing is actually not made with fudge believe it or not, often it's a chocolate buttercream but there are other recipes. There are a few problems I can see with using traditional cooked fudge as a cake layer: