Mastery level, skills and str/int ratio

[Monco]

I messed up my character a little bit. I'm level 10 at Lighting, but I don't need that skill anymore, since I'm training Cold and Bow skills.

1) Can I lower my mastery level if I reset the lighting skill back to 0?
2) How much would that cost?
2) As I already said before, I'm training Cold & Bow skills. Should I make Str and Int even or not?

Thanks in advance.