Are the American Indian factions playable?

[Szlachta]

Are the American factions added in Kingdoms playable in the grand campaign with this mod? Including the Aztecs? If not, is there a mod that allows this?

[Stylix]

Are the American factions added in Kingdoms playable in the grand campaign with this mod? Including the Aztecs? If not, is there a mod that allows this?

Not with the default installation. However, the Aztecs can be made playable with some slight modifications.
In the HHB (Heroes, Hordes and Behond) sub-mod for v4.2, the Aztecs and the Appachians are/can be made playable. I can't remember what the initial faction selection is because I have modified my starting faction choices. However, it is very easy to change and only requires the ability to use a text editor to make the changes. Any non-playable faction at the start can be made playable, but some of these factions, like the rebels and the Papal States can have some unexpected results.
From the FAQ thread:

Well you can make them playable easily by editing the descr_strat.txt. Look in the mods\kingdoms_grand_campaign_mod\data\world\maps\campaign\imperial_campaign folder. Open the file up with notepad and at the top delete a faction entry for example portugal under campaign playable, and add it to the list in end nonplayable instead. Now remove Antioch from end nonplayable and put them in under campaign playable, done! Save and exit.

This applies to any factions just substitute the faction desired with Antioch in the above directions.