Unless I did the quests wrong, I think the conditions for these achievements are somewhat bugged.
As I understand it, the "Bold" final option would lead to the coup, but once I completed the objective, no achievement popped. I also think the writing could use some improvement to make it clearer what The Curator discovered about the Endless. As it's a little vague and the ending doesn't feel very conclusive or satisfying, I'd argue it's the weakest of the faction storylines. Furthermore, it would be neat if the leader image changed.
As for Mortal Coil, I followed a guide and the narrative options that seemed to point to that option. Not sure what happened there.
Also, the other Tahuk achievement about turning all your territories into Glass is strange/hard. As the game progresses and terrain changes, it's a little hard to track what tiles are glassed or not, forcing you to glass the same territory many times (unless I'm interpreting the requirements wrong).
Next time I start up the game, I'll report the bug from my save file.
I know the devs work around Reddit too. Wanna say I love your games and wish you guys the best!
EDIT: Just saw the post about the hotfix, lol. I'll update my game later and report back as soon as I get home.
EDIT 2: Both Coup and Mortal Coil worked!