I chose Lander Lebeda to be my envoy, but there was the half orc Kassil, and the older woman to choose from as well. I suspected that the two I did not choose would represent the interests of their own nations, not mine, which is why I chose Lander, since he seems to be more impartial. Can anyone confirm this, or have any horror stories of their envoy's behaviour later in the game?

He is a traitor.

^^ you should not tell him that! :P

But yes. its better not to take him.
The problem is, the other guy is a military guy, from whom you have at least 3 better ones at some point.
And this old lady? i don´t know. She could be the better solution.
But yeah, everybody is better then this piece of shit.

Choosing Lebeda hits you with a malus later in the game that makes two of your advisors useless. There should be a way to get rid of the malus, but apparently it doesn't work due to bugs. So it screws you for the rest of the game.

In my opinion, the game suffers from lack of advisors in general. Playing a good guy, there is only one available councilor (Tristan) since I chose Lebeda. And it is possible to lose him too.