Ingredients vs. Temperature Control in Chocolate Fudge

In reading through a LOT of different approaches to making fudge, I am wondering about how the non-sugar ingredients affect the issue of temperature control (both heating AND cooling) and, therefore, the results in the recipes.

First of all, a question about heating the sugar solution - am I correct that in bringing a sugar solution to the soft-ball stage temperature, what you are doing technically is boiling-off water to the point where the sugar concentration is appropriate for the re-crystallization properties desirable in fudge? So, the temperature gives you an indication of the concentration of sugar in solution and different concentrations behave differently when cooled, is that right? Basically, the temperature is documenting sugar concentration achieved by way of evaporation.

If that is true, then I can buy that in heating the sugar and milk or cream or condensed milk to make fudge, the temperature would still be dictated by sugar concentration in the water part of the milk-product. So if we think of a a milk-product as water + solids + fats, the sugar concentration is still reliably documented by the temperature since the sugar will dissolve in the water of the milk and is (basically) unaffected by the solids and fat content. Is that, more-or-less correct?

Continuing along this same line of reasoning, would it also be correct to assume that as long as you add to the pot only ingredients that contain NO water, that you would preserve the integrity of the relationship between the temperature of the ingredients and the concentration of sugar in solution? So heating the chocolate or marshmallow (or whatever) with the milk/sugar solution won't fundamentally alter the relationship between sugar concentration and temperature?

Then what about adding ingredients during cooling? Most recipes where you heat all of the ingredients together (except butter and vanilla) instruct you to let the mixture sit undisturbed until it cools to (like) 110° F - I thought that the goal there is to have the sugar in a supersaturated state so that crystallization will occur rapidly when you stir.

But some recipes instruct you to add other ingredients after bringing the sugar/milk to the soft-ball stage - I would think that adding ingredients during cooling could induce a temperature change that could screw-up the supersaturated state and give you grainy fudge. Most of these types of recipes contain marshmallow. So, since marshmallow is mostly corn syrup, is the corn syrup controlling the re-crystallization properties of the sugar? If that's true, do you need to be careful with a marshmallow fudge during cooling at all?