In the case of Evita, Klausner would go on. She was always there as she was also in the ensemble. As was Nancy Opel, the original understudy, who became the alternate when Klausner left.
In London, however, I think the only alternate Eva who was also in the ensemble was the first, Susannah Fellows, since she was the understudy when it opened, at which time they were not planning to have an alternate. Fellows became the alternate as well as the understudy around three months into the run. Then a year into the run, Marti Webb became the alternate. Webb was way too far along in her own career to also be in the ensemble. And as far as I know, it stayed that way with the later alternates, even though I think they were all near-unknowns.