If you are looking to enhance or change the flavor then injecting can be beneficial. I normally do not inject pork butts when I am smoking them. Every once in a while I will if I am looking for a different flavor profile.
No. Only thing that helps with tenderness is finding that sweet spot where the connective tissue breaks down. Before that, and it will be tough and taste dry. Wait too long afterwards and you'll end up with either mushy pork or tough, dry pork.
I have never brined a pork butt, and I have injected in the past. I find that injecting pork butt isn't really worth it. If you cook it low and slow it really should be moist when finished. If you want to change the flavor of the pork the inject away, but as said before you have to be careful not to wait to long. Mushy pork sucks! Best way to impart new flavors to you pork in my opinion is to come up with a rub that you like, or use a finishing sauce (yumm). Just my thoughts.
I take a small cooler, slightly larger than the butt, and make a brine of kosher salt, garlic powder, onion powder, bay leaf, brown sugar, rub and any number of other things I find to pull out of the spice cabinet. Simmer the ingredients so that they disolve and open up their flavors. Add ice water to cool the liquid and cover the butt with the brine. I fill the rest of the cooler with ice, put on the lid and leave overnight to absorb the liquid / flavors.