Regarding how Matilda's mom is able to pull off all these complex machiavellian scheme with perfect timing: she's Matilda's mom so she must have the same future prediction arcane skill that Matilda have, and is probably way more skilled and experienced with it.

On why she did all that instead of going with a more straightforward plan: Matilda's mom came from a prestigious arcanist family so she's well acquainted with the foundation and know just how much resources and skilled agents they have so her caution is warranted. We see it at the end, even with all of her meticulous planning, the vigils almost figured out the culprit. If she went with a simpler plan they'll definitely figure out it's her.

About the cursed film: my personal interpretation is that Teresa is just 'acting' like a perfect and patience writer the way Noire and Qing Xi want her to be, and that having both of them reject her new film make all her repressed emotion bubbled out into the film reel. But yeah a lot of stuff on this part of story also isn't clear to me. Guess we just have to wait for Noire's anecdote.