Using 12% is fine. It may have become a little less potent after 2 years in the fridge, but you can still try using it. Follow the protocols for using 35% peroxide, but use three drops for every one drop. If you have a serious illness, then it might be worth it to get fresh 35%, so that you know for sure it's as potent as it should be.
You can take extra antioxidants if you want (but not at the same time as the peroxide), but it isn't neccessary. Most of your body's antioxidants are made by your body itself, taking supplemental antioxidants likely only makes a small difference. At the concentration of peroxide used, your body's antioxidants are more than adequate for protecting you against oxidation of your cells.