Note it must be POWDERED peanut butter. Peanut butter has fat in it and the beer will spoil.
I would not do the powdered sugar at all. And no, you can't add it in after. It would be giving the yeast food, and activity would go crazy. If it was in bottles, they would explode.
Two St. Pat's HMEs and a Robust LME will get you close to 8% ABV.
You can add chex at the end of the boil. I'd start with a cup and adjust up or down in future batches based on how it tastes to you.
You can use cocoa nibs and powdered peanut butter in the boil as well.
The powdered sugar...well, I have no idea. I can't imagine much of the sugar flavor will make it past the yeast.