I'm trying to understand why 先 is not the best choice to use in these sentences. Is it wrong to use it in this way?
I'm going to assume you mean 先 as in さっき. It's usually written in hiragana to avoid confusion.
I think the first sentences are just fine. The × one sounds like spoken language and ○ one sounds more like written language.
In the second × sentence, though, さっき would not work because it's used for things that happened "just now" and ten years ago is not "just now". Also, the の there is not correct, it's only used when さっき is used to modify a noun or a noun-phrase. You could say e.g.「さっき書いたメール」 for "an e-mail [I] wrote just now" though.