Hello...I'm about a week ahead of you; had my C5-7 ACDF Aug. 7. I have a Miami J collar to wear all the time, with the Philly for showering. My doctor did say I could lay on my side, but since you only have a soft collar at night, I would recommend you have plenty of pillows to keep your head from tipping sideways, or forward/backward. A lot of people sleep in recliners the first few days/weeks, especially those of us with a hard collar on at night. I have an adjustable bed and always sleep at an angle, but even with that I sometimes find the recliner more comfortable. I had donor bone, so I don't have the donor site issues you probably have with your hip, which of course may affect your ability to sleep on that side. As far as neck movement, mine moves slightly inside the collar, especially if I don't have it tight enough

I haven't done any major movement with it yet, but I'm not very much more experienced with this than you! I wouldn't recommend any flexion, extension, or twisting at all at this point, though...especially since you don't have any internal fixation such as plates.
When I saw my NS last week, he wanted me to start taking muscle relaxers on a regular basis, which has definitely helped me sleep more soundly!
Good luck on your recovery. Glad your hospital experience is over!
Donna