Question $1$ of $2$: What does this phylogenetic pattern suggest about the speciation process of these species?

As each island appeared from the ocean, some individuals from the older nearby island dispersed to the new island and became isolated from the original population.

The big island in Hawaii was once larger, but eventually broke up into multiple islands forming a chain. The large population of flies in the original islands was carried off in each new island, become isolated from each other.

Because the size of the islands are very different, they can contain different population sizes of flies. This causes genetic drift to be variable among them and increase divergence. That is why the brach lengths of the phylogenetic tree are very different between islands.