How about:
At some point in Blake and Alexis's marriage, Blake had an affair. The woman didn't go to Blake first--she went straight to Alexis, hoping to destroy the Carringtons' marriage and eventually claim Blake for herself. Instead, Alexis already had some sort of incriminating information on the woman and basically blackmailed the woman into aborting the baby and leaving town. Only the woman didn't quite follow instructions. Blake never knew about the baby, and Alexis has assumed all this time that the woman had an abortion. In Alexis's recent "research" in her plot to take Carrington Atlantic from Blake, she found out Amanda existed, but residual hatred of Amanda's real mother (and perhaps residual worries that she might become another obstacle in her scheme) kept her from bringing Amanda into the mix. Plus, she knew that Amanda would never side with Alexis. Alexis simply chose instead to use the Fake Adam angle, since she could control the situation better. So Alexis only in the past months learned about Blake's illegitimate offspring, and is holding onto that information in case it comes in handy.
This would allow them to make Amanda any age they want, and her real mother could be a character from the original series (tying her into the new show in a new way). It would also overcome this idea that Alexis raised a child while in exile, since they've characterized her decade or so away from Atlanta as a non-stop party. Instead of being the apple of Alexis's eye, Amanda will hate her more than any of the kids, since she will know Alexis ordered her mother to abort her because she was "an inconvenience" to Alexis, and Alexis's actions denied her the opportunity to grow up knowing her real (and real rich) father. Any efforts of this Alexis to come across as a doting mother would fall flat, much like Anne Matheson attempting to behave like a loving mother to NS's Paige on Knots Landing always set viewers thinking "What's she up to now?"
