If the problem is connected to the retina iPads - it's got something to do with device resolution, that for some reason the simulator isn't simulating.
Since I was creating the video on device, I was making a 2048x1536 video on the retina devices (and 1024x768 on the non-retina devices). Apparently this is just too many pixels for AVExportSession
to handle, or the iPad to play properly, so it just threw a variety of vague error messages at me on play, or export. Recording at the point resolution, rather than the pixel resolution seems to have resolved the problem.
The simulator appears to be a red herring since it has the comparatively limitless resources of a healthy mac at its disposal, rather than an A6.