It gets a lil complicated when you're "priming" and it's still fermenting. If you're going to pasteurize, you should just try to do that when it's at the sweetness you like. The only reason I'm chiming in is because if you're trying to end up with something sweeter than not, it's much simpler to stop it where you like it. By adding dextrose (or any sugar for that matter), you're getting the yeast party started again. Just wanted to throw in my two cents and I apologize if I missed something while skimming through the thread.