Before we get to the opportunity before Core, we need to discuss the seeds. Specifically, we need to discuss what Core knows about eating food from the Underworld. When Rhea hands down the deal, the stipulation is that Core cannot have partaken of any food from the underworld. This is something that Demeter does not even balk at, and it is something that even Hades’s gardener can understand, though he shouldn’t even be privy to the terms of the deal.
When a compromise or a deal is made, consideration is given to both sides. But the compromise that Rhea hands down would be completely one-sided if not for the provision about the food of the underworld. Given how all sides react to this particular clause, this is not a surprise or new invention, it’s boilerplate.
In fact, given what this applies to, it may reflect a law of nature, like gravity, or Voltron as Defender of the Universe (Lion Voltron, not Vehicle). The underworld is almost always a one-way trip. People check in, but they don’t check out. Heroes are the exception, such as Odysseus and Heracles, however, they need to cross into the underworld while alive, and are restricted to what they can and cannot do.
The very nature of the underworld is that it hold onto people who enter. They become part of the underworld, and are inseparable from it. So when Core eats the seeds, which are part of the underworld, she, too, becomes part of Hades’s realm, which changes the ball game.