for "Green Acres"LIKE it when Lisa,Eb and Mr.Kimball(who were kind of bright and not as dim-witted at the beginning),acted stupid and Oliver was always grouchy(in the beginning he wasn't too grouchy,not as grouchy as he was in some episodes-especially the latter ones)?

I think what they did was find out what made each character who they were on the show and overplayed it towards the end. For example, like you said Mr. Kimball is the forgetful one. The writers saw that that had worked and decided to keep adding to it.

I don't believe "test audience" even existed in the 1960s.

Really? When did test audiences become established.

My guess the 1980s at the earliest.