Sundering and specific character questions. [Spoilers]

[MrThinker]

Do we have any information about how the Sundering might have affected Midgarsormr or Omega, if at all?

Also, as an aside, did we ever learn why Emet-Selch, Lahabrea, et al., were not among the Sundered?

The answer to both of those questions (outside of speculation) is no.

For the 1st question at least, we have a somewhat solid theory barring new evidence, in that the Sundering only affected the original Star and did not extend to the surrounding universe, i.e. go past Hydaelyn and Zodiark's "field of influence". Midgardsormr and Omega, who both are of alien origin, would thus not be affected by the Sundering, especially if they arrived after the event. This goes some ways to explain why Middy and his First Brood are so powerful, because they are unsundered.