In a Gameplay and Story Integration example, the influence of Yvonne/Layla over the target characters is so big the even "real life" players of the game have it hard to win Penelope's route. Players can still easily buy the hidden ending, but they still have to pay a "high price" to get it.
As for why Penelope needs to max out the affection of a love interest, doing so is what allows the target to resist or even become immune from Yvonne/Layla's influence, preventing her from killing Penelope, the only individual left of the Ancient Mage bloodline able to stop her for good.
It's shown there's many types of poisons, even slow acting ones that take days to kill and are hard to detect. So considering that Layla-Ivonne's is trying to invoke a Deliberate Injury Gambit to frame Penelope and gain support for herself by drinking from a poisoned cup, why does she pick such a deadly poison and not have an ally nearby with the antidote? Because she would both likely survive being poisoned given that Layla-Ivonne has a Healing Factor and is nigh immortal to anything but the Mirror of Truth, and it causes the nicely dramatic Blood from the Mouth seen in other Otome Games that would maximize the sympathy factor. Also, she may have arrogantly assumed Winter would use his anti-toxins to help her. She didn't expect Penelope to take the poison instead, however.
