Trundle doesn't know her secret, he just decided to be her ally in exchange for her not killing him. Now he's waiting for the opportunity to backstab her.
I might still be confused. Is Lissandra known to be the leader of the Frostguard and the Ice Witch known to be evil, but nobody knows they're the same person? Or is it just out in the open now that the Frostguard aren't what they seemed or what?
I think it adds to her faction's identity that her only champion ally isn't a true one. It also gives their storyline room to be advanced.
It might if it was better done. It feels really rushed to me- some troll thugs his way past her guards, allies with her, then goes back and becomes king. If that was cutting out more nuance, then yeah, it might be a good story. As it stands, it feels downright random.
And the only changed lores I find weak now were mostly weaker before the changes. I'm looking at you, Gragas.
I liked how Olaf was from a mystery land nobody knew about, and was trying to get home but also waaaaaaaaay too savvy to let the Summoners trail him there. Now he's just kind of an infeasibly stereotypical viking. Wants to die in glorious battle, can't because his rage is too strong. Which is also kind of stepping on Tryndamere's toes.
The exact manner in which it's done seems really, really weak, too. He's not old, he's not decrepit, he's not retired, but his literal plan was to go out and get himself murdered on the spot. Even for a people obsessed with glory and viewing death as inevitable, that's really pushing it.
Gragas... I dunno. Old Gragas at least had refuge in mundanity- he didn't have any grandiose ambitions because he didn't really want much out of life. New Gragas has a purpose and a faction, but the former's kind of asinine (and worsely-explained than Old Gragas' magical leakage mutations) and the latter's pretty weak.
The rest are kind of borderline or positive, I suppose. Sejuani's probably better, Ashe's changes are fairly trivial, Trynd's changes are nice in that they're intriguing and cut down on the number of Noxus Killed My Family Want Revenge-motivated characters, but also make him seem less directed and organized and more like a lone berserker, which I'm not fond of. Nunu I preferred as a human yeti to a Disney Freed-Beast/Rebellious-Kid duo, but at least he has more motivation than nothing now, I guess.