Nothing to do with the truncated file problem but a bit of an experiment using your 751 layer frx.xcf file
It is possible to generate a video as described earlier, using some utility (anitools) to write the layers to individual files then generate a short (2 or 3 frame) video from each using ffmpeg. The problem is trying to get some frame timing using ffmpeg, you might want a 2 frame video but you are more likely to get 1 frame or 3 frames.
However as an example:
https://dl.dropboxusercontent.com/u/10581986/final.avi 21 MB with some very strange effect in the middle for those small layers.
Attachment:
from-pngs.jpg [ 104.92 KiB | Viewed 2450 times ]
The 751 xcf layers becomes 2002 video frames @ 20 fps = 1min 40 sec duration, a little longer than an equivalent .gif run. You can easily change the duration by changing the frame rate, say 15 to make the video longer or 25 to make it shorter. The number of frames remains the same.
Just a comment on the image size. 1218 x 914 is not a standard video size. For avi, width and height should be multiples of 8. For YouTube, they will re-encode your work at the drop-of-a-hat if it varies from their requirements and wreck it. Stick with 1280 x 720, for audio - mp3 is ok.