Do we now why she wants to meet an Ultra Beast? I mean besides Gladion apperantly saying she's wanted to see one from a young age? Despite the anime not directly saying Mohn's name we still see a wedding picture, they purposely blinded with light to not show who's she married to. I don't think obession with Nihilego caused her to hate her kids but instead she couldn't take them "rejecting her love" and leaving so she simply disowned them, the toxins made it worse but her obession wasn't the direct cause.
You keep saying "if you've played the games" but the thing is that they've drastically changed Lusamine's character from the games. In the games, Lusamine investigated Ultra Wormholes to find her husband and slowly went mad to the point of abusing her children and dressing them up like dolls. Gladion states as much, and in the original SM, Lillie practically yells at Gladion for "leaving her alone with mother". The toxins made things even worse, but she didn't show signs of her abuse towards her children just because they ran away. Sure, she went crazy further because of that, but it wasn't the impetus.
In the anime, though, it's explicitly stated that Lusamine has wanted to see Ultra Beasts since she was a child. Gladion said as much in SM 49. So her motivations are genuinely to protect and care for her children, rather than her abusive counterpart. To say she's still evil, or has been all along, simply wouldn't make sense and, like Akkipeddi said, reek of a forced nature that no one really wants.
Also, I thought it was Silvally that caused Lillie's fear, not Nihilego?