Danfling,
As another poster stated, I believe Judge Lowell last appeared late78/early 79 as he was not seen at Dan Stewart's funeral.
Granpa Hughes was married to Mrs Kopechne in July 75 and didn't that all happen offscreen? So I presume Santos Ortega was probably unwell and not up to appearing before the show expanded. I imagine the funeral was offscreen also (maybe Penny,Edith and John were mentioned as having been there)
Ethel Remey/Alma was written out due to illness and replaced with Dorothy Blackburn until Alma could be written off. Ethel Remey didn't pass away till 79.