It seems very unlikely that the A5 used in the 4s was a fallback plan. First of all, they were already getting a huge speed bump by going from single core A8

It seems very unlikely that the A5 used in the 4s was a fallback plan. First of all, they were already getting a huge speed bump by going from single core A8 to dual-core A9. So existing iPhone users would have been happy. There was also no foreseeable external competitive pressure at the time, everyone knew that there weren't going to be any S4 phones for 6-9 months, and that Tegra 3 would be a joke. They were also still on a 45nm process, and the A5 with dual-core A9s was already huge (120 mm^2). They would not have wanted to increase that at all, even if they had a working design of their own. What would have been the options? Seems like it would have been either reducing GPU resources, or dropping a core. Hard to see either of those being worth it. Finally, one could argue that they main plan was their own core@32nm, and the A9@45nm was the fallback if there were process troubles. But these days there seems to be a strong preference to alternate new designs with new process nodes, rather than doing both at the same time. And in fact this conservative option is exactly the one Apple picked next spring: the iPad refreshes were a mix of 32nm parts (simple shrink) and 45 ones (higher clock, more GPU). If Apple weren't willing to risk going to 32nm even for a design iteration as conservative as A5 -> A5x (and the process was clearly working for them with the A5 shrink), the idea that they would have tried for a radical new design + new node 6 months earlier doesn't seem credible.