Interesting Donna Reed article.....

[Mustard]

Then if Reed were successful as a recast, she was destined to pass away in 1986 anyway, so there’s another prob....

Yes. Had Reed carried on in the role, she'd have done little more than half the 1985-86 season at most.

 

[Chris2]

The government would have asked Ellie to go to South America to help establish a ranching industry down there. She would have been presumed dead in a helicopter crash. Then, an imposter, played by Barbara Bel Geddes, shows up saying that she’s Ellie.

 

[Luke_Krebbs_Ewing]

The government would have asked Ellie to go to South America to help establish a ranching industry down there. She would have been presumed dead in a helicopter crash. Then, an imposter, played by Barbara Bel Geddes, shows up saying that she’s Ellie.

Just after that happens, Wes Parmalee arrives & claims he's the real Miss Ellie!!

However we subsequently learn that he is in fact Jock Ewing.