I'll try to explain this episode as much as I can for the confused ones, due to how weird the whole the Tomoe Arc itself is. This whole season & few good episodes, & most aren't related to this & are almost filler. This one is the weirdest of all cause it's part of story, yet feels worse than a generic shounen filler episode.
Here's a semi-TL;DR explanation:
The past:
>Mariel & Sean fall in love
>Either, Allen may have raped Mariel & knocked her up. Or she Mariel may have cheated on Sean.
>Mariel runs away to Revonland
>Tomoe shows up & gaslights the fuck out of her.
>Sean calls, Allen interrupts, then another one of Tomoe's students (either Catherine, or some other random student) shoots them both & commits suicide to hide tracks.
>Mariel freaks out, gives birth, & dies (possibly from maternal death) while Tomoe watches from a distance.
Meanwhile in the present:
>Finn finds Mariel's tapes & podcasts them.
>Lupin & Mattea listen in.
>Lupin & Mattea decide to go to Revonland.
>Mattea says someone failed.
>Lupin tells Finn to breathe.
>Lupin & Mattea leave.
>But what does it all mean? Here's the BIG explanation:
Mariel was in love with a fellow high school student Sean, but another student, Allen, raped her & left her pregnant with Finn. So her father (Finn's grandpa) sent her to Revonland away from her home & hired Tomoe to be her tutor as she was "someone who could help alleviate traumatic memories" which prety much increases the rape possibility more. Somehow, Tomoe (who also tutored Mariel's mother) sort of brainwashed Mariel into replacing Sean with Allen in her subconscious, therefore making it seem in her mind that she loved Allen & thus forgetting about the rape. Tomoe however also went ahead to have one of her students* kill both Sean and Allen & then herself. Perhaps she wanted the boys dead so they would stop calling Mariel and endangering bringing her real memories back (as it sort of happened when Sean called, probably what Mattea means when she mentions that Tomoe "failed").
>Maybe the shooter was Mariel?
No, notice the freckles & hair color.
>Maybe the shooter was another one of Tomoe's students?
The shooter was probably "Mariel's friend" Catherine. It would be weird to namedrop her & show that Mariel doesn't remember her if that played no role in anything. Remember how Tomoe mentioned she went to America first & talked to the three there? Catherine was probably another one of Tomoe's agents, & Mariel probably did die in childbirth. Given the way Tomoe was lying it's plausible that "Catherine" was not her friend at all. She doesn't show up in the photo of the football team either.
Finn went catatonic when she learned of the shooting & the girl's suicide so there's a possibility that it could mean that some of Tomoe's conditioning transferred to her when she listened to the tapes, making her believe she was Mariel, which was reinforced by her grandmother's dementia (potentially also a conditioning?)
>Whats's the said conditioning?
It looks like Tomoe hypnotically conditions her students, maybe to turn them into Manchurian agents. "Daijoubu" is probably a trigger word to enter trance. IIRC, other episodes also have this, I think she also said that repeatedly in other flashbacks.
Basically the Winter Soldier thing from MCU.
There's also a possibility that Anna (the dementia grandma) may have been conditioned into her dementia too. Tomoe also taught her.
Also, before you say, yes, streaming your private family history is very retarded, She was basically asking for Tomoe to hunt her down. But then again, she didn't knew what Tomoe exactly is.
One more thing here to be food for thought:
Here's the date of the shooting:
Here's Finn's podcast account showing her B'Day: