could we get omega marks from borg and undine elite missions

Would give a reason to do things at the elite difficulty besides better chance at loot and difficulty. Although every Borg mission should drop Omega Marks and the higher the difficulty, the more Omega Marks. Undine missions might be not available for Omega Marks since it seems like Omega Reputation is strictly for the Borg and there might be a new reputation eventually for the Undine.

You mean for completing the storyline missions on Elite difficulty?

I think any missions that involve Borg should give some Omega Marks regardless of difficulty. The higher the difficulty the more you can get.