The lich class existed before the template system was implemented. Your observations about it are correct. Basically, the DR in the class is very heavy, at the expense of spellcasting. The demilich template is not as heavy on DR but loses no caster levels and has, as you point out, a lot of spell-like abilities, most of them as many times per day as desired. The class is the better defended, but the template is far, far better as a spellcaster.
The leadership anomaly you have noted is caused by the demilich template. The leadership scripts are fairly old, I think, and somehow they react to the demilich as having a higher level than the actual character level, so a 21 level character with the demilich template can summon a 30 something level cohort.
I would say the DN was coded to not be able to take craft wondrous item until level 19. The situation is strange because of the PRC template system. In PNP becoming a lich and advancing is expensive in terms of gold and experience (as it is in the PRC for the lich class). Becoming a lich for free at level 20 is thus a big bonus for the DN in PNP, and so it happens at a higher level than it would for someone paying for the lich status. The PRC is set up to enforce this level restriction as per PNP, but the situation is fundamentally different than in PNP because of the way the templates function. In the PRC right now, yes, a DN gets to be a lich "for free" at level 20, but any evil character with 11 caster levels of the same type could become a lich "for free" at level 11. THe DN PNP advantage is thus actually a disadvantage in the PRC system, since the DN has to wait 9 levels longer to become a lich than any other spellcaster. I don't know if it is possible for the acquired templates to be coded so that they check for gold and experience and subtract some when applied, but such a change would restore the balance. For all I know, though, it could be a big coding headache and thus not worth the trouble.
