Amy:
I feel a little "101" here, but what is the bullet point implications here - I kinda assumed fused = no movement anywhere.  The SIJ might not move much but lose that micromotion and some macromotions get effected. What is that lateral marco motion used for, is probably a better way to ask my  question.
 
Jerry Hesch
Amy, just after I posted it I scrolled through my You tube videos and made the discovery some time ago, just did not recall! So there is another video on same topic with CT scan evidence. Yes fused left sij means no movement in the left SIJ. I do believe that every biomechanical expert would be very surprised to see that a one-sided SIJ fusion significantly restricts pelvic side-glide mobility. Pelvic side glide is not thought to be a pure motion in the sij, but rather very minuimal sij motion and more motion of the lumbar spine and of the hip joints.  Place your hand on the sacrum and try to glide it left and right, tell me what you find. lie the person on their left side and compress the right ilium and spring test it. What do you feel? I will  explain.

Jerry Hesch
The lateral micromotion is a functional motion during gait, lateral weight shift, etc. It of course becomes a macromotion during such movements. The test for this motion is very relevant as some clients have it restricted, yet it is treatable with Hesch Method and it is a preventive model. Side-glided pelvis' even if subtle alter weight bearing throughout the kinetic chain, especially the lower extremities, and of course as you know; will create a distal compensation such as at the occipitoatlantal joint, which in time becomes symptomatic and non responsive to "adjusting the upper cervical spine". Does that answer your question. Micromotion is such a fundamental property of joints like the SIJ, so very relevant to test if one is using terminology such as "joint". Otherwise one is talking about the bony pelvis as it moves on the hips, which is NOT sij motion, despite the popularity thereof. I believed such for many years, but could not ignore the burgeoning research.