implausible
these supply chain rumors are almost never correct.
I'm holding in my hand a 4th generation iPod touch, internally a 3GS driving a retina screen.
The ipad mini is 4x the volume of the device, with space for 4x the battery. The display technology has been perfected and power reduced since 2010. There are no new challenges about fitting this technology into the iPad mini.
The reason the original wasn't retina has less to do with engineering than with marketing: it wouldn't have hit the cost they wanted, and would have eaten too much into the higher ASP, non-redesigned iPad. And Apple's continuing marketing strategy is not always to release the best products, but to release products that make the previous ones look old. If they launched with retina, the upgrade this year wouldn't have seemed as compelling. Now they're going to update it with the 4s's camera from 2011, and a retina screen from 2010, and have an all new looking product for 2014.
That's the secret to Apple: the feeling that they're creating newness.