
So to sum it up, she was a maid working for the ml but then because she knew that the saintess was going to poison the ml, she knocked it out his hand but then got mistaken as an assassin. Because the poison could only kill those with high mana like the ml, Ciel drank it to prove that she wasn't the assassin but then somehow became younger. To repay her for saving his life, the ml made her his fiance and asked for her to get adopted to the Visennas even though he hated them because he knew that they would be able to find out what was wrong with Ciel and get her back to normal. Sorry for the long explanation and if I was confusing but I'll try and answer your questions as best as I can :)

Hiii sorry for the late reply I was sleeping. ╥﹏╥ Um so I think these may be spoilers so
SPOILERS:
I think the saintess tried to poison him because she was having an affair with the emperor and since Ferze (the ml) is a threat to the emperor, she poisons him for the emperor. As for the villain part, I'm not exactly to sure but I think it's more like people see him as villain like while the Visennas are the actual villains of the original story but of course, they're not the villains for this story.

I thought it was because she loved him when they were younger (since they were paired together at birth) but he failed to notice that she loved him. Later on he would have to do his duties for 6 months as the grand duke & she came to hate him for failing to realize that she loved him (basically all that love turned to hate).
...so she was a maid who drank? Something meant for poisoning ml? Saved him and became a kid again instead of dying? And is adopted to the ml rival family...? Is that correct?