1) It needs to read the disc each time. While the entire game is just downloaded and the disc never used for gameplay, the disc check is there to prevent the exact scenario you describe. Pure digital versions of the 360 games however, can be shared like One games via setting each other's consoles as their home console.
2) No. The enhancements are exclusive to the X. Though even without enhancements, 360 games load and perform better on the One than they do on the 360.