IMO, things only get complicated when you are looking for what seems to be irreconcilable outcomes. It is easy to back sweeten cider or wine. You simply allow the yeast to fully ferment your cider dry. That should result in a gravity reading of below 1.000. it might be .996 or even .994. Make certain that the gravity is absolutely stable (traditionally , three readings over two weeks). You rack the cider and allow it to age until it is bright (absolutely clear). You then add K-meta and K-sorbate (both are needed and you need to dissolve the K-meta in a little water as it does not dissolve well in alcohol). After say, 24 hours you can add fermentable sugar and any remaining yeast will not be able to ferment it.
However, you have now for all intents and purposes removed any possibility that any straggling yeast cells will bud or act on sugar so you cannot now carbonate using sugar.
How much sugar to add? That will depend on the amount of alcohol in your cider, the amount of acidity, the level of tannins. The best way to determine how sweet THIS batch of cider should be is to bench test.
Bench testing means that you take say four or five known quantities of unsweetened cider to which you add known but different quantities of sugar. You dissolve the sugar in each glass and taste. If, for example, your sample size is 50 CCs of cider and your sugar is say 5 gms. You add 5 gms to the first glass, 10 to the second, 15 to the 3rd, 20 to the 4th. If 5 gms is not sweet enough but 10 is too sweet, you redo this test. But now you add say 7 gms to the first and 9 to the second. If 7 is not sweet enough but 9 is too sweet then you know that about 8 gms per 50 CCs is what you are looking for.
For the sake of argument , let's say you have 3785 CCs of cider (that is about 1 gallon US) . Divide that by 50 to determine how many sample sizes you have (approx 76) so you need to add 76 *8 gms of sugar to your volume of cider to add 8 gms of sugar to every 50 CCs (or about 608 gms) .. (These numbers are made up: I suspect that adding the equivalent of more than 1 lb of sugar to a gallon of cider will be cloyingly sweet )
It's simpler than it might appear.