So I'm pretty sure the second letter is from Autonomy. And here I will explain why. I will attempt to explain how Trell, Pajti, the Sand Lord, are all seperate pieces of Autonomy. This does bear some similarity to my Spirit Realm thread, which is in my signature. But the most important thing is this WoB:
https://wob.coppermind.net/events/256-oathbringer-london-signing/#e8605
So I think that by splitting up her mind, Bavadin became more powerful. How? Investiture. This is the hack here. She split it, and used the investiture to grow a new personality. A persona. An Aspect. Almost like harnessing the ability of Investiture to create sentient life. Then, with these personas, she is able to grasp more of this Autonomous investiture than she could if she was just one mind. It's a hack. One that appears to have created a collective. And with these personas, she will spread her Autonomous doctrine across the Cosmere.