You don't have to restart from the beginning of the citadel unless you really want to do the quests. From a later save you can simply run back citadel entrance and backtrack to the imp quest, as Lady Rain described.
Be warned that the summoning doll skills can not be unlearned. If you want to play around to test them, save first so you can reload if you wish.
You do succeed in undoing the soul forge at the end of the game, so the hero is alone for the after game battlefields. The act 1 summoning doll could be handy then, since it is the only one that can use ranged weapons (crossbow). If you don't really like the BF dungeons, then the next doll will work just as well to hold loot or trigger traps.
