[X] Plan Without A Fancy Name
-[X] Let Jeyne and Dayne disappear for a while, claiming that the same person that tried to assassinate Dayne had abducted Jeyne and cursed her in some fashion. She is in an undisclosed location, seeking healing while Dayne helps her. This place is unsurprisingly SD, which will be the important scandal for everyone to focus on, for it is unlikely that people won't be able to guess that much.
-[X] After a few weeks, Dayne returns, his honor intact for having helped the lady in dire straits, but she will remain away for a while longer to heal. (Read: Have the kid in peace.)
-[X] From there on it's their choice. If Jeyne wishes to become the new Lady Dayne, they can play at having fallen in love and married. The exact birth date of the child doesn't need to become public knowledge, thus curbing the rumors somewhat. If not, she can return and claim the child to be from someone entirely unrelated, or do the switcheroo with her sister.
-[X] No matter what, have Tyene discretely explain Imperial marriage law to her before she makes her choice.