If you barely warm the sugar and then add it to your wine will the sugar further breakdown making your wine sweeter than you want? In other words will it raise your SG level?
By "inverting" sugar, you break down the structure to a simpler product. In the beginning it will be easier for yeast to consume. (Think of it as cutting up your steak as opposed to eating it as a whole).
When you 1st add the sugar to the pan you have wine and a pile of white sugar in the pan. Stir and you now have granulated sugar suspended in the wine as it warms.
After a few minutes you will begin to see white streaks like a comet as the wine swirls around your pan. Stir frequently. Some granules will still reside in the center on the bottom of your pan.
When all of the sugar is disolved from the bottom you may see granules clinging to the side of your pan. You barely have a flame on the pan and the wine is just now reaching room temperature.
Now all the white streaks have disappeared but if you look closely and look across a "sheen: of the wine you will see some air bubbles and sugar residue in the wine. Almost done.
While stirring if you let it slow you will see a difference between the center of the pan and the outside of the wine. When this difference in texture/color are gone turn off the heat and remove from the burner. The pan will be warm.
Let it go a few more minutes. It takes sugar 7 minutes at least to convert (break down). It is this time I do other things to my wine, ph readings, acid tests, finish racking, etc.
When you no longer see white streaks and there is no crusty edge your sugar is inverted and fully disolved. Add it to your wine and blend. Take a sample and get a gravity test.
You are correct if you do not fully invert the sugar and it finishes disolving overnight then yes your wine will be too sweet.
I have never had this problem. Get in some good lighting and do a small batch in water as a test. You will see what I mean as each stage is completed. Even if it is not completely finished it will as it rests a few more minutes.
All in all you can do this in less than 10 minutes. Be sure to write down how much sugar you add at each addition for your records.
Keep in mind wines with a greater % of acid will require more sugar to have the same taste of those with a lesser %. Think of it as iced tea, one with and one without lemon.
From the wikipedia def. of inverted sugar
"Inverted or invert[1] sugar syrup is a mixture of glucose and fructose; it is obtained by splitting sucrose into these two components. Compared with its precursor, sucrose, inverted sugar is sweeter and its products tend to remain more moist and are less prone to crystallisation. Inverted sugar is therefore valued by bakers, who refer to the syrup as trimoline or invert syrup.[2]
In technical terms, sucrose is a disaccharide, which means that it is a molecule derived from two simple sugars (monosaccharides). In the case of sucrose, these monosaccharide building blocks are fructose and glucose. The splitting of sucrose is a hydrolysis reaction. The hydrolysis can be induced simply by heating an aqueous solution of sucrose, but more commonly, catalysts are added to accelerate the conversion. The biological catalysts that are added are called sucrases (in animals) and invertases (in plants). Sucrases and invertases are types of glycoside hydrolase enzymes. Acid, such as occurs in lemon juice or cream of tartar, also accelerates the conversion of sucrose to invert."QUOTE