Hey Caponer, your energy and diligence are amazing!
As for Meksiko versus Veksiko ... this is just a matter of mutation. The word Meksiko in Cornish will always be mutated after a (m → v) as it will after many other words. On reflection, you may be right about Sita Veksiko rather than Sita Meksiko. It depends whether Meksiko is feminine or not (or at least it does in Welsh; I'm tolerably sure the rule's the same in Cornish). If you have a possessive construction with Noun + Noun, and they're both feminine, then the second one is mutated. México, Mexique etc are masculine in Romance languages, so it might be in Cornish as well (although I think Mexico the city is feminine in French). Ken George is coy about this, however, as for some infuriating reason he doesn't mark the gender of place names (at least not in the Gerlyver Kres).
On the other hand, having a Veksiko redirect (as we do now...) is all to the good. (The preposition yn, however, is not a word that causes mutation, so it should be yn Meksiko rather than *yn Veksiko.) Oll a'n gwella, ☮ yn-medh Owen. 16:42, 16 Meurth 2006 (UTC)