You not seeing the Aqua in Xion =\= they're not similar. Enough people did spy that.
When did I say that Xion was made by Sora? It was talking about how Namine was created.
Yes, which is why I find it ridiculous you're trying to tie Xion to the person who makes this all so complicated - Kairi.
Kairi can't have a Nobody on her own. Xion would have to be that much of an uber special complicated Nobody to be Kairi related.
If she's Aqua related, it solves most of the problematic issues of her birth, being mostly - that Kairi couldn't have created her on her own, whereas Aqua could've easily done so, when all she'd have to do was stay alive until Xehanort created the first Heartless/Nobody.
And hey, seeing how he has her Armor in the Room of Sleep he himself made - who's to say Aqua wasn't a test subject in the experiments that gave us Heartless and Nobodies to begin with, hm?