Tell us about yourself!
You add 5% of extra fat than it takes to theoretically react with the lye. This is a safety measure in case of errors in measurement or calculation. Since oils are a natural product the percentage of individual fatty acids varies. A little unreacted fat is not a big problem, but unreacted lye still present is.
It does seem to be acceptable in common circles, but as a chemist, I highly recommend against it. If you get zapped that means you have destroyed cells. I like the cells on my tongue just the way they are. Best practice is to use a proven recipe, super fat by 5%, measure carefully and try it out on your skin. Don't risk your tongue cells. In the comments section of the article I posted one woman wrote that she had in fact hurt herself by doing it.
There's no reason why you can't use the blender for food afterward. I realize you are speaking of safe practices, but there's nothing in here that is truly toxic. Lye is caustic, but not toxic and it will wash away. It's funny that you say that and then talk about tasting it. No, don't taste it, use a pH meter.
No, do not add more water, use KOH instead of NaOH.
It's not picky. They are right, calcium carbonate does not produce a gas, flammable or otherwise in water. You know spelling nazis are considered trolls though.