To number 1, often headstones are used as part of research for families. Such as determining if someone is a Cohen or Levi.
Accordingly, in my humble opinion, I would think the appropriate inscription should be what your late Mother-in-Law’s actual name was.
As far as query 2, I personally do not appreciate double headstone, certainly not ones erected while one of the people were alive, so I will recuse myself from responding.