The duration of time it takes to remove them would depend on a slew of variables. How much help they employ / request, the presence of the undead, the nature in which they intend to remove them, how hard they retaliate for the attempts and etc. Point is, a more dynamic approach to their removal would honestly give them a less benign feeling.
The Undead are already a defined entity. To remove powers, add powers or give any major changes would require some variation of logical cause. How the undead change is of no major concern, to be completely honest. I'd just rather see possibility of removing them via RP and for them to be treated as what they are, an entity for the entertainment of the players.
And to actually define a choice, I'd be for 'A' but, a very player driven A.