a) sure, but that is fun.
b) Chaos has to be a huge temptation to those civs that are neutral, but can be turned, otherwise it would indeed be more realistic for civ-leaders to play happyhammer.
But if choatic/neutral nations have no choice over the matter the intro text needs to be rewritten to exclude the suggestion of having that free choice.
c) yes. agreed on that. the neutrals only. (and only those that would be tempted.)
Me likes deadliver's suggestion of norsedwarves for Norse (and berserkers!).
They could be usefull flavour units. A potentially chaotic civ such as Norsca, should have a hard time fighting rebellions and incursions from chaotic cults, more so than the more southern oldworld civs. Turning to chaos should turn these extra assaults to a strength adding free units. maybe a choice can be offered each time one such extra incursion occurs.
"Our rebelling coutrymen have grown powerfull with their new gods! shall we join them and share in this power?"
-Never! We cannot betray our friends the dwarfs. To arms, together we crush this evil!
-Yes! Come dark brothers, let us pillage the South! (all dwarf units disbanded, chaos marauders added.)
Werewolves and -bears are also very Norsca UU.