After a flimsy hot gluing of the board in place, the MPU9150 took its 1st 2 batteries of flying. Results were drastically improved over having the board under the battery. There's still a bit of error because the board isn't perfectly flat, but it manages to recover. The current or the flexing of having it under the battery was the problem.
There was 1 source file for the MPU9150 on http://permalink.gmane.org/gmane.linux.kernel.iio/4339, revealing undocumented register REG_YGOFFS_TC. He never uploaded the header files.
With the full Marcy 3 software & the crystal, the MPU9150 only reads at 80Hz.