Basing the 1D5 and 5D3 around the same sensor would be an interesting departure, in terms of the positioning of the 5D, but more interestingly in terms of achieving economies of manufacturing scale, due to the number of 5Ds that Canon sells.
3] Both new 1D and 5D will have the same sensor. No 6D, no 3D, no 2D, no 8D, no 4D, no any other "entry level" FF DSLR, a used 5D or 5D2 is your entry level. 5D2 should not be discontinued once 5D3 is released, big price drop expected ($2000?). I don't think that Canon could drop the APS-H just to make another, parallel FF line. It's more about sharing the same sensor and adding more features while keeping the sane price.
That would also preserve sales of the 1Ds line, because photographers who really need super-high resolution would not have the option of the 5D line as an alternative.
If this happens, I would expect a sensor that is shared by the 1D5 and 5D3 to be in the region of 27 to 32MP - definitely higher than 18MP.
Has Canon painted itself into a corner here by allowing the 600D and 60D to reach the same sensor as the 7D well before Canon really needs to replace the 7D? It would be difficult for Canon to update the x0D or xx0D lines, especially with a new sensor, before releasing the 7D2.
5] New APS-C sensor next year for all new crop bodies - 7D2, 70D and 650D.
What this all does still beg the question of is whether Canon would consider a lower MP full frame body. That is probably dependent on the economics of producing the sensor. There are probably a lot of enthusiasts who are still put off by the price of the 5D2, but who would pay a price somewhere above that of a 60D, maybe as much as a 7D for a full frame body.