Pretty much no yeast is going to leave much residual sweetness behind, apple juice is just too fermentable.
I only prime with corn or cane sugar, ever, because anything else is too unpredictable and offers negligible benefits flavor-wise, but that's a personal decision. There's not really any certain way to know if 1/2 cup of concentrate is enough, too little, or too much.
Whether or not 1 pound of corn sugar is enough depends on how alcoholic you want it to be. Keep in mind alcohol has a drying effect on the palate, so that dextrose is going to make it seem marginally dryer than it is.