So, uh... Alice taught Marisa magic as a kid, then got attacked by... something and erased Marisa's memories of it... to what end?

No idea, but it looks like it didn't stick.

It somehow looks like it was Marisa who injured Alice.

Okay, what the hell just happened?

when marisa was a kid
alice taught her magic from a book
somehow marisa hurt alice
alice, didnt want marisa to feel depressed about that, so she erased marisa's memory. alice knew magic so she gradually healed, though it seems she is hurt badly
teenage marisa, when looking at the old book, remembers about that day

So it looks like a merger of Alice past when she became a yitch and left Makai, with Marisa mother's passing.