I agree that the iPad's 264 PPI being right between the 260 to 270 PPI of what doubling of the current MBA resolutions would probably be more ideal for usability, but from the standpoint of economics of scale I'd think being able to use more of their MBP sheets would save them more money.
I doubt that. Apple sells tens of millions of iPads per quarter, it'd take 2-3 quarters of Mac sales to match that kind of numbers. In the last quarterly report, Apple sold 14.1 million iPads versus 4.6 million Macs. It's not known what's the breakdown on those MBA vs. MBP but MBA is generally their top selling model.
It makes all the logical and financial sense for Apple to reuse the same known and reliable technologies in the iPad for the MBA retina model rather than the more expensive MBP screens, especially as they need to make the 999$ price target.
They already have at least two different screen suppliers for their iOS devices, they just have to expand the contract to include the MBA screens, which would include the same amount of bulk discounts they already have now.