Don't worry about niacin at this point. The pedialyte will be fine. Sounds like he needed your help, and maybe he just needs a little extra time to warm up and get his muscles working right. If he was in the shell muxed up, he may be stiff and sore and take a little longer.
Yup, any old sharp scissors will do. Run them under piping hot water for about 30 seconds to sanitize them, or just boil them on the stove for a minute or so if you want them completely sterilized. The belly is just his yolk still absorbing--it's not unusual in a slow starter. He will probably not be really active until it absorbs all the way. Knowing that it's not absorbed though, I'm thinking you should probably minimize handling of him, as that part of him will soon be on the inside, and you definitely don't want to introduce infection there. Also, he still needs the damp environment of the incubator to keep the yolk moist, and it wouldn't hurt to wrap the belly in a damp cloth until it absorbs all the way. Just make sure everything that touches him is clean clean clean.
Good luck with him. I sure hope he strengthens up for you!