Actually, I'm stupid because I just glazed over everything. The OP took 4.5 gallons of juice plus 6lbs of fermentable sugar and selected a yeast with a 12% alcohol tolerance. It's going to get to 12%, kill off the yeast and then leave a ton of unfermented sugar behind. If 2lbs of dextrose in 5 gallons of apple juice typically yields 8.5% ABV (depending on what's in the juice) then the OP has roughly enough sugar for 25-26% ABV. After the alcohol kills the yeast @12% that's gonna leave a TON of sugar behind.
I vote it tastes like fire with enough sugar to burn the enamel off your teeth! If this has already been done, maybe let it ferment to 12%, rack to secondary then make a starter, add a nutrient smack pack and repitch with something like Wyeast 4946 which has a high alcohol tolerance (18%), is good for restarting "stuck" fermentations and deals well with a lot of fuel floating around. I don't know what that would taste like, but it's definitely a yeast that could pick up where the 4766 left off.
I'm literally laughing out loud because I want to know how this turns out. Come to think of it, I bet it could turn out alright. I don't think you could bottle carbonate it without serious trial and error, but it might make for a really interesting overly sweet white wine after a LOT of aging. You could also use it to make Black Velvet. I bet Guinness never tasted so sweet or had such a big kick in the pants!
Go for it! Seriously. Why not?