If we use our foreheads to butt football (the round one) over an extended period of time, will we unable to co?

The damage done to your brain is really all over your brain, it isn't really just where your head was hit, it spreads out quite easily.

footballers use their heads because they have brains in their feet

Not unless you get concussions. Although every time your head hits something you do lose a few brain cells. However, you lose brain cells doing pretty much anything, so does it really matter?