There doesn't need to be any flats to apply the adjustable wrench to. If you leave the nipple in the drum, as you rotate the wrench, it will bump into the side of the nipple and it can't rotate further without the drum turning. I've done this dozens of times and it works great. No. A little 6 inch wrench will be more than enough to turn the drum. Drums typically have a 5/16" or 3/8" thread on them and they aren't torqued down hard. Just enough torque to line up the nipple with the hammer is all that is required.