Top rack for me but I also don't think you need to brine a butt at all- it will be flavorful and tender with the fat content in it.
all you really need to do is trim and rub. I used to inject and marinate and found the only difference was the mess I made.
225 is perfect but I think mesquite is too harsh to do the whole thing at, a bit at the beginning and then fruit wood for most of the cooking.
I assume you are doing a bone in boston? Are you doing the whole thing?
I honestly usually smoke until it gets to 170ish, wrap with juice, honey, beer, whatever, and move to the oven at 225 to finish. You really wont get anymore smoke flavor after that temp anyhow.