Do you have any sources to back up any of these assertions, e.g. that this is what Sumerians called 'Nibiru', or that they had astronomical records of this/an object?
If I remember correctly, the "Planet X" / "Nibiru" stuff is largely tinfoil-hat territory (see the link dalke posted, as well as other articles [1-4]) - a hodgepodge of speculation, conspiracy theories, pseudoscience and -history, and literal interpretation of myths - as made evident by a cursory search on Wikipedia.
Planet Nine, though, seems to be something other [5] than this mess entirely.
As for the doomsday myths, I find them equally ridiculous whether the planet in question would be number 9 or 10 on our somewhat arbitrary planet-system.
[1]: https://en.wikipedia.org/wiki/Planets_beyond_Neptune
[2]: https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calend...
[3]: https://en.wikipedia.org/wiki/2012_phenomenon