My belief is that there was more than one culprit: the person who orchestrated the poisoning and Oliver
The Fairylord also mentioned how much fun it is to watch people, on ch2p12. Some things said there might fit Oliver.
That's why I also believe more people have been at work here, even if they haven't been fully aware of the deeds of the others.
MF also isn't off my radar. He gave a plausible chain of arguments, why he wouldn't be the murderer...for the reasons, that he has given himself. That's just blindsiding people, with asumptions they made themselves or they would most likly make, if they would think about it. Just tell 'em the most likely way things might have gone down, lead them to the train of thought you want them to board and bon voyage. He didn't gave conclusive evidence, that he wouldn't have done it for different reasons or couldn't have done it in a different way. What if for a reason unknown to us readers right now, John couldn't ascent the throne after her death anyway? MF would still be the next King, even without the brother's death. Or the brother gets killed anyways in an failed offensive against the enemy. (Of course someone would make sure.) Or the marriage between the bother's now finace, than to be widdow, and the MF afterwards is already a deal made behind scenes. No, MF is still suspicious af.
Also, maybe the poor maid was killed to blame the poisening on here, while somebody else poisend her and goes unnoticed now. Even if MC wasn't expected to return from the dead, killing her as a scapegoat, just to make sure in case the poisening gets discovered, isn't unthinkable. Eventhough, the thing with the maid has been handeled sloppy. Or maybe she noticed something was wrong with the tea, called out to the wrong person and had to be disposed off on the spot.