Spidergland not only neglected her daughter's kidnapping, she actually instigated it.
Due to her paranoia, Spidergland most likely wouldn't be satisfied with merely keeping her daughter Spidervenom imprisoned in the Dark Realm, since there's always the chance Spidervenom could escape and get her hands on the Azoth sword. It's possible that she struck a deal with the Cerulean Land to extradite her daughter's imprisonment in order to make it even less likely that the Azoth sword chooses Spidervenom. This could explain why she's so willing to go along with the Cerulean Land's obvious bad faith diplomacy, since they could blackmail her for being complicit in her daughter's kidnapping.The Giant boss at the end of the game is only a fragment of the original Giant's power.
The lore in the Grand Dark Temple states that the Giant defeated the Dark Mother, but that was after it was summoned by the three Peacemakers, who likely had more human sacrifices available to power it up. The one summoned by the Cerulean King only had a handful of sacrifices and can be beaten by the party without much trouble, meaning it is likely weaker than it could have been. Future games in the series could feature stronger versions of the Giant with greater amounts of sacrifice.