SEH Build Process Questions

1. For flange-mount electrical boxes - when the box is fixed to the wall - must all the screws be already used on the flange, or can a temporary attachment be made?
2. If not, is there any other allowable means of temporary attachment that others use or may be used? (fishing for efficiency detail)
3. For the heat pump disconnect, is a nonmetallic push-in connector acceptable as a a clamping means for 10/2 cable for the incoming line?