I can understand why it might feel like Telsin's powers came out of nowhere. I think some of beats are a little looser in these books since they aren't a "main" series.
But, we did know that she had been given powers. Wax mentions that she is a leecher once they are hit with the cube. I also didn't see it as a surprise since we learned in the temple that the Sets top people, like Edvard, were spiked. So it seemed obvious she was. At the end of the book either Wax or Wayne even states that it makes sense that one of the powers she chose was healing because who wouldn't.
As for Sazed not knowing what would happen to the southerners, between the Mistborn trilogy, these books and the Secret History of Mistborn we know that the Shards are limited in what they can foresee. He was also doing all this in the first few moments of godhood. He didn't take time to explore every action and every consequence.
And maybe he did know what would happen in the south but what was he supposed to do? Sazed just put the world back were it was before TFE. Mucking around with the natural order caused huge problems in the first place. Seemed like the safest, best choice.