Jimmy-128 says >
"Lombard" and Vera survive, she because she really is innocent, he because he's not really Lombard
You're right about who survives but I don't think it's for the reasons you state.
I feel they survive because they figure it out (well, Lombard does) before they can be eliminated. And, according to the movie, the last 'man' standing would want to kill himself instead of face the law and be hanged for having killed the others. The real murderer dies so there's no one around to kill them. The murderer had no idea neither of them was guilty of the crimes they had been accused of committing.
Woman, man! That's the way it should be Tarzan.
[Tarzan and his mate]

Yep. Remember when the play was produced: early 1940s England. Specifically, England that was getting the hell bombed out of it on a nightly basis by the Germans. Christie felt that a dour, downbeat ending was the last thing her audiences needed, so she changed it.
Since there was a very real possibility that Christie would not survive the war, she also wrote two "final" novels for her main series characters, Hercule Poirot ("Curtain") and Jane Marple ("Sleeping Murder"). Since she did survive the war, the novels weren't released until the 1970s. The novels themselves are excellent, but they do feel a little odd. In "Curtain", one gets the sense that we're still in the 1940s from the way the characters speak and behave. In "Sleeping Murder", a couple of character who had died of old age long before suddenly show up again, hale and hearty.

Thank you. Wow so they changed the characters completely. And that's the ending Agatha Christie rewrote for the play?

If you really want
"Lombard" and Vera survive, she because she really is innocent, he because he's not really Lombard.