Metaphor in The Odyssey
Notice how even after Odysseus leaves Calypso’s island, she hinders his progress towards home. Homer uses this metaphor to show how the power of the gods can reach anyone regardless of whether or not the god is physically present. Considering the time this was written, we can surmise this to be a reflection of the cultural values of respecting the unseen gods.
Anticlea means this both literally and figuratively. It's difficult to reach Erebus, because it's so far from the mainland, but it's also difficult for Odysseus to see it, as a man whom we're led to believe has many years ahead of him.
If the geese in Penelope's dream represent the suitors, as Odysseus claims, then her feelings in the dream suggest that she would accept a suitor to remarry. This is not surprising that after years of doubting whether Odysseus was alive and after being pursued by this large group of men, Penelope would start to feel attracted to at least some of those who want her hand.
These sudden changes are not meant to be taken literally but rather metaphorically. It's unclear whether Theoclymenus sees these things because he's a seer and can foretell the future or because the suitors are behaving so strangely that it's as if the world is shrouded in darkness.
A gadfly is a species of fly well-known for biting and goading cattle. In modern parlance, it also refers to an irritating or difficult person, so that if we liken the suitors to the cattle, Odysseus becomes the gadfly who, at least from their perspective, torments them unfairly.
If Eumaeus was trying to bolster Odysseus' spirits by mentioning his father, Homer is trying to undermine their courage by suggesting that even the strong grow weak and feeble. This fight will not be as easy as Odysseus hoped, and perhaps not as righteous.