After the torpedo is fired, it explodes in the ice, but it was a plot point that they didn't have explosives for their torpedoes.
Compass magnets are extremely weak. Using a compass magnet to try to grab a bug with a bellyful of metal would be completely ineffective.
The crew deal with the infestation of bugs, then spent a long time resolving their issues with Nemo; meanwhile, they have not fixed the hole in the hull and the Nautilus is still filling with water.