They don't share the same integrated GPU; the MBA is a HD5000; the 13" Retina is a HD5100, the 15" Retina is a HD5200.
The major practical difference between the HD5000 and HD5100 is clock speed; while they have a similar theoretical cap, at full speed the HD5000 uses 22W (more than the TDP of the chip it lives in!), so in practice generally operates at far lower speeds.
They could probably get away with a HD5000 powered Retina thing, but screen power consumption would still be a factor. IGZO should deal with this sooner or later, though.
Not so much in battery tech. It's really holding back a lot of hardware innovation on computers and also holding back the viability of EVs for many consumers.
The Retina display in iPad Air is much more power efficient compared to the retina display in 3rd gen iPad due to reduction of backlight LEDs and move to IGZO type of panel.
The current Macbook Pro/Air situation is fairly elegant- it's a simple tradeoff between portability and resolution when you choose between the two 13" Macbooks. And it's not as if you get shafted by that choice, given how similar the machines are aside from their displays.
As for power, I deliberately didn't go into detail, but the short answer is Broadwell.
And when Broadwell chips roll out they will be placed in the Pro just as easily as the Air. They're both on Haswell right now.
Product differentiation matters to Apple. I just don't think a slightly slower processor and a 0.01" thickness cuts it.
To create the new Retina MacBook Pro, Apple took out the spinning HD, optical drive, and soldered in the battery. But they also put in a Retina display--as well as the more powerful GPU and larger battery it requires
The current MacBook Pro IS the Retina Air in everything but name only. It's the thinnest and lightest laptop that Apple can make with a retina display.
In summary: the hard part of a Retina Air is not the pixel density, the hard part is the power. Any blog post that does not address that question is not adding much to the conversation.