Unfortunately replacing a 2nd molar with a wisdom tooth is not an option at your age. Braces can only move teeth a tiny bit and they aren't meant to move a whole molar tooth over. You have two basic options here: Leave it alone or place a dental implant. Leaving it alone is a bit risky, as the tooth above your missing 2nd molar will slowly move down and create issues. Dental implant is really your best choice here.
Regarding your second question, you don't have to remove your wisdom teeth to start braces treatment. Of course, it's better to do so, but not mandatory in most instances (infection abscessed wisdom teeth are the exception and they must be removed right away). Just be sure to wear your retainers religiously if you choose to keep your wisdom teeth for the time being. Hope this helps and good luck!