Will Botox be effective for a square jaw with a prominent chin?

I used to chew gum frequently.
Maybe because I like chewy things like squid and sometimes eat them, my square jaw is quite prominent.
Has anyone experienced good results with Botox for a square jaw?

Botox needs to be administered every six months to be effective.
Does the jawline become less severe with each repeated treatment when the effect starts to diminish?
Since muscles deteriorate if not used, I wonder if the jaw might do the same.
If you use your jaw muscles less, won't the muscles in other parts of your face develop?