And as you say on your site, accelerometers measure linear acceleration, not rates of rotation. I haven't read further into your design, but what I can tell you from contract work my employer exposed me to, you're not really going to be able to 'determine orientation' from acceleration. The project I am referring to was a weather buoy project, that used accelerometers to transmit information about sea states. The problem is that as a buoy pitches, the accelerations have to be corrected for orientation (not the other way around!). The purpose of using a magnetometer to correct the accelerations for orientation was to decrease power consumption and cost.
Regardless, the IMUs you've listed seem to package this all for you.
But, I do want to say thanks for keeping track of the parts you've bought. I had started doing the research to build something similar and basically got bogged down with the many options out there. All the parts you'd found seem very reasonable as far as specs and cost.