It was interesting to see how much your original concepts for Maze Burrow diverged from the final! I can only wonder what we would've ended up with if you had persisted with one of the two, instead of going back to the drawing board...
I remember having seen your game on one of the Feedback Friday threads. 1-3 wasn't much of a roadblock for me, but the boss level was a big spike in difficulty — I don't remember if it was reused in the final game or not. I'm glad that the feedback of myself and the other users helped out, though.
It's interesting to note that some of your favourite puzzles were ones that you, at first, thought to be impossible. A video from Game Maker's Toolkit comes to mind, which gives examples of puzzles that result from a contradiction. The player wants to do two or three things to complete the level, but one of those makes the other impossible, and it's up to them to find a way to bypass that contradiction. The levels with warps and paired blocks are a good example of this in action, at least to me.
I'm glad that the ranking stats are only cosmetic, Otherwise, I would've never, ever unlocked world 6 or 7's secret levels.
It's too bad to hear that Maze Burrow didn't sell very well. I barely heard anyone I knew talking about it, which would reflect the low response rate from the reviewers you contacted. Hopefully, more people come in future to play this underrated gem.
It was good of you to make the source code public. A comprehensive modded version of Maze Burrow could be fun, adding in new gimmicks... though, that's also the sort of thing a Maze Burrow 2 would be expected to provide. Maybe it could even make use of those blocks that only you or other blocks can go through — though, like you said, it could be tough to think of good puzzles for them.
Is it true that all of the rocks use the same sprite? I thought that the rocks that killed you were more red, and the ones that merely stunned you were brown. Considering 7-B (Rock Barrage), I also doubt that a majority of the rocks in the game are harmless. ;)
Overall, while it's too bad that your game didn't sell very well, I think you did a great job on it. I'm looking forward to whatever games you may have planned in future!