Hi, sorry for the late reply.
I am not sure why you other post didn't get saved.
You are correct about the instability of the adjacent angle when going beyond 90 degrees.
We are working on code to fix this. Both none AHRS and AHRS.
Try and look around the net for AHRS code for the LSM9DS0. There is plenty out there and AHRS will fix your problem.
Mark --OzzMaker.com --