Backflash will actually kill anyone of 2 HD or less. Echo Fist is a neat concept, but a death sentence, since you can easily be getting up to three rounds of dazing or more at relatively low levels. Time Flash is harder to judge, because I don't feel like it's necessarily overpowered, but it's definitely better than other similar maneuvers from the core book. The stance also seems much stronger than Child of Shadows, which is a relatively solid 1st-level stance.
The idea of maneuvers that scale with HD is very intriguing. The 20 HD abilities might be a little silly, though, just because so little of the game takes place post-level-20. Maybe something like "if you have the ability to use maneuvers of level X", where X is this maneuver's level +5. So if you can use 6th level maneuvers, your 1st level ones get better, making them potentially useful to prepare again.