This arguably falls into the category of "could be answered with a simple dictionary search", but I think in this case it would be hard to know what to search for, because it's made up of arguably three different things (
To answer your question, the original translation of "heaven sent offer" is pretty close to accurate. The reason is that
願ってもない is a set phrase meaing exactly what one was looking for, or precisely what one wanted.
Edit: since the question has been updated to be about etymology, I will attempt to address that.
First off, it's worth pointing out that the
ない here is
無い and not a contraction of
いない. It's not "something not wished for", but "something that doesn't exist even if wished for".
I can't find anything that explicitly addresses the etymology, but dictionary definitions provide some context. For example, From kotobank:
When something wished for but unlikely to actually happen luckily appears
You can find similar explanations here and here. Basically, it seems like this is expressing the idea of something being desirable but typically out of reach.