Under extreme conditions of heat, diamond could be vaporized. Because diamond is a non-molecular material that is held together by strong #C-C# bonds, conditions would have to be exceptionally fierce.
Given that #C(g)# would be much less dense than #C(s)#, we could not accurately depict diamond atoms, carbon atoms, on the same scale.
A simple molecular substance is simply a discrete molecule, i.e. #O_2#, #CO_2#, #N_2#, a material with definite molecularity, i.e. boundaries. Of course, you could choose many others. A giant covalent substance is a non-molecular species, whose particles are individually linked by strong covalent bonds in an extended lattice with no molecular boundaries. Quartz, diamond, graphite, are examples.
As for question 4(b), this is a rather nasty one. MY own feeling is that #C_60# is a molecular species, because it contains discrete molecules with definite size and structure. Certainly, an organic chemist can make bigger molecules than this.