You can return to the island, though there is not anything wonderful there you need (a couple teachers, half decent sword...). Return to the lava where you came in, to get back.
Unfortunately, the first time you leave the imp village you get summoned by the necromancer. Due to a bug, if you leave by entering the battlefield, then enter / leave the BF again, you get returned to your original location, rather than the one in the abbey.
To finish the abbey, you will need to reload a previous save. You can continue without this quest, though you will miss the experience, loot, teachers and an item that would give one character a permanent mana boost.
