Q1: Don't all people say grace before dinner at the same time in a family which believe God? Why did Mrs Simpson ask so?
Usually one person says grace and the others say "Amen" when he/she has finished saying it.
Q2: And is it usually 'guest' who say grace before the dinner?
Not always, but it can be.
Q3: do the honors= ?
Say Grace. (in this context).