Great writeup! A good reminder that in reverse engineering you can get pretty far just with static analysis and a bit of educated guesswork. Also a fantastic demo of using statistics to get a foothold into how the files are structured.
If anyone is interested in some similar research, I did a video writeup using very similar techniques for reversing the graphics format of a cutesy platform game - https://www.youtube.com/watch?v=e0x7KU4wLGw