Does anybody know how freely the two ends of the mavic props should swivel around relative to another? I know they fold up and come together but they seem to rotate around the center so freely sometimes. Occasionally when the engine is powered down theyre not even always directly across from each other. I hope they always in right position when flying lol? Should they ever be tightened?

The fellow on the DroneValley YouTube videos was demonstrating in one of his videos how he thought you could test if the blade rivets are too loose or not.  Basically, hold the extended blade horizontally by one of the blades.  If the other blade droops down, then he felt the center rivets might be too loose.  Not sure what DJI feels about this.

You can rotate the props and just make sure you install the props correctly.
Besides, you can turn on the aircraft even if the props are fold up.