If the tilt meter output is erroneous above 50 degrees, there is probably an error in the code. Here is a clear and comprehensive analysis of measuring tilt angles using an accelerometer: http://www.freescale.com/files/sensors/doc/app_note/AN3461.pdf As the document explains, there are some difficulties with angular coordinate systems that need to be understood.
What is the min and max raw data value to be expected if it is set at sensitivity of 2g
Just record the output as a function of different orientations, and it will be obvious.
Consult the data sheet for bandwidth considerations. As for the low pass filter, adjust alpha until the results are satisfactory to you.