Well that's where - and God, it feels so wrong to say this - but that's where Byrne is the solution. Donna becomes this awesome playmate that Diana had for a single summer, and then lost - which means that from then on, not only did she not have a playmate growing up, she knew what she was missing, which makes it that much more poignant. (It also, I think, makes a lot of sense that a kinda-lonely Diana would have tried to *do* something about it, and it gives her interesting symmetry with Polly as well, who after all did the exact same thing.)
Slot that into the post-FC retcon (or at least one of them, depending on what title you're looking at), where a just-barely-adult Diana discovers the kidnapped, still-teen Donna and rescues her, and that casts that childhood decision to create Donna in still more bittersweet a light (because she created this person and promptly massively failed her). Meanwhile, there's no real interference in the uniqueness of Diana's creation myth, her effect on the amazons, or her own adolescence, because Donna's there and gone again and isn't really *part* of Diana's life until her adulthood.
It's also really the best way to explain the stark difference in dynamic between Bruce-Dick (father-son) and Diana-Donna (older sibling-younger sibling) despite the fact that the age gap is the same.
science lured me away...
Yeah, I was lured by *everything.* English Major was kind of the ultimate expression of "dammit I cannot narrow my interests down!" - writing gives you an excuse to learn about just about anything, because if you want to, say, write a story about a supervillain trying to build an orbital laser, it behooves you to know how such a thing would actually *work,* and then next week you can write an episode of one of sixteen million medical dramas and research some kind of crazy-ass cool disease instead.