[[WMG: Sheltem will be returning as the BigBad of ''[[VideoGame/MightAndMagic Might & Magic X]]'']]

Well, the trailer did give a rather unsubtle hint. Unless the guy speaking is an imposter pretending to be him, I'd say it's pretty likely.
* WordOfGod says unambiguously no, at least beyond InNameOnly. The quote was just a nod to the old games: ''X'' isn't ''allowed'' to take place anywhere but Ashan, and that entails certain limitations -- among them the fact that Ashan is pure fantasy, without any science fiction elements.
* Utterly jossed with the release of the game -- the BigBad is a native Ashan entity. Sheltem's only appearance is a number of mentions as 'the Usurper' in a series of books that retells the plots of the previous Might & Magic games in somewhat veiled and poetic forms.

[[WMG: The Duchy of Tatalia and the Kingdom of Tatalia co-exist]]
According to Heroes Chronicles, Erathia used to rule (poorly) over Tatalia. It's possible that when the Revolt of the Beastmasters overthrew the Erathians and established the Kingdom of Tatalia, it didn't break off the entire area that used to be called Tatalia by the Erathians, and the rump Erathian Tatalia eventually became Markham's Duchy of Tatalia (which, perhaps notably, in its in-game presentation only has a comparatively small area of wetland along its southern border).
* Seems pretty likely. The scenarios "Borderlands" and "Greed" in the Spoils of War campaign of the Restoration of Erathia show that there is a good deal of swampland well inside the borders of Erathia. While those regions aren't called the Duchy of Tatalia (they're instead called Marshland and Hillbridge), the Duchy of Tatalia could easily exist there.