That sucks about the GamePad AC adapter especially since the battery isn't great and the console is so intimately tied to the GamePad (I don't think you can even use the eshop without it). Nintendo also doesn't sell those in stores, not that you'd want to spend $25 for something that was faulty. I've had to be extra careful with my GamePad AC adapter because... cats.
The only issue I had exchanging games was a few months ago when I went to exchange a $40 game with a $35 game at Toys R Us. It's a longer and more complicated story than I feel like rewriting, but you can read about it
here if you'd like. The short version is that I ended up saving $8 additional because they had to honor my receipt and they made the mistake originally. There wouldn't have been an issue had they exchanged the game correctly the first time.
In any case, I did ask the Target clerk about this on Sunday just to be sure and he said he's done this before (I didn't tell him my whole master plan, I merely asked "in case I wanted to exchange"). As long as the games are unopened, I have the receipt, and it's within the exchange period, they have to honor it. I can definitely see why you would be weary of doing this. The whole endeavor is mildly complicated, but I like free things. I really wouldn't be surprised to see this sale or one like it again in November.