Are juices too salty for a low sodium diet? Will using unsalted butter help?

This is the sort of question that probably ought to be discussed with a doctor and dietician. There isn't that much salt actually absorbed from the brine- but it could still be more than a strict low salt diet would like. You can also brine using a salt substitute, since chemically it acts the same way, so far as the turkey is concerned. I personally don't bother basting a turkey, because I don't think it accomplishes anything except extending the amount of roasting time required and heating up the poor basting cook. The fat and moisture doesn't really get through the skin to the meat, and I prefer crispy skin from allowing it to dry out a day before roasting. But IF I were going to baste, I would probably prefer to use ghee- which is unsalted and water free. I've never found the drippings from an overnight brined turkey too salty for gravy making, but I also use an unsalted stock to add to the drippings for the purpose. But if your low salt diet needs to be followed strictly, you need to talk to your doctor about your best options. Which would probably involve a fresh turkey that wasn't injected or brined at all.