May someone tell me what Wonder Woman issues state that Donna is Diana's mirror image and Cassandra is Zeus' daughter, please? And where it was revealed that all Donna's origins are "true", please? It's important.
For Cassandra Sandmark, it's written in the notes :
"Zeus was revealed to the audience as Cassie's father in Wonder Woman (Volume 2) #181-183; Cassie suspected it for some time, but did not learn definitively about it until Wonder Woman (Volume 2) #217."
The "mirror image" origin is in Wonder Woman (Volume 2) #134-135. Not sure if it's the first occurrence, though. As for the "all origins are true", I guess you're referring to The Return of Donna Troy ? (more specifically issue 4) where she remembers her life on each Earth ?
According to Titans her prime earth origin is the first one in Wonder Woman comics: she is made of clay (an analogy to Wonder Woman origin pre flashpoint). The "retcon" that made her origin more similar to her original origin (one of them at least) in the first numbers of Titans was then discovered to be false memories put in her to heal her trauma of being "an artificial being" and her villanous past in the Wonder Woman comics...
All the same, how she can be made of clay recently in Wonder Woman comics, and have a past with the titans when they were young is one of those things that Rebirth never explained...
Pre flashpoint origin mess was explained during Final Crisis to be because she is the merged version of all pre crisis versions of herself... so she had all the pre and post crisis acumulated origins at the same time XD
Donna was always one of those characters that are a continuity nightmare, but at least that was somehow addressed many times and almost explained... Not like the Hawkman nightmare XD