1. No; energy band model is obtained for a crystal with no defects. Defects are introduced as perturbations. Defects are randomly placed and individual placements are of no consequence.
2. The splitting of the energy levels are due to "exchange integral" or resonance process which is due to the fact that electrons are indistinguishable.
3. At low temp, the electrons will always go into the lowest available energy state (this makes the system lower in total energy); at 0K, the energy of the system should be the lowest.
4. In simple text books, the individual orbitals are not shown separately but if you look up journals, you will find them all the way...
5. For He molecule and atom are same; you mean why we do not have He2 molecule, right? That is because 1s and 2s energy levels are VERY different. Energy of a structure with 1s2 and 2s2 both filled is too high (this is a crude description) compared to two separate atoms.