You’ve asked what may be three separate questions, so I’ll focus on your question about what contributes to the unity of the poem, particularly in regard to the poem’s development of the characters of Scyld the Scefing and Beowulf.
My first thoughts on this question are on what seems to work against a sense of unity. There are number of so-called “digressions,” in which the storyteller seems to become sidetracked or lose sight of the main character and main plot altogether. The very opening of the poem may be considered one such digression. This epic opens by talking about Scyld the Scefing (I’m using the Gummere translation right now; other translations often use other names; yours may call the first person mentioned Scyld Scefing or maybe even Shield Sheafson), and it then moves on to talk about Beowulf, Scyld’s son, who still is not the title hero of the epic, even if he shares the same name. Other digressions include mini-stories about family feuds and various queens. The title hero doesn’t even appear until well into the epic, in book three. The epic’s seeming chaotic structure can indeed prompt us to ask about its unity.
What counters this tendency of the epic to stray off topic are recurring themes, terms, and images that surround both Scyld the Scefing and Beowulf. The first might be the concerns about what makes a “good king.” Scyld the Scefing is praised, at the end of his brief appearance in the prelude, as a model for what it means to be a good king; the Old English text exclaims: "þæt wæs god cyning!" (“That was a good king!”). Similarly, Beowulf might be seen to develop, in the course of the story, a fuller understanding of what it means to be a good king. It’s not just about acquiring fame; the good king has to share his wealth, forge alliances, honor his promises, and defend his people above all else. A second point of unity is established by the repeated references to the mead hall, that central building of early Germanic culture, whether it is wrecked or saved. Warriors and kings are praised both for destroying their enemies’ buildings and for preserving their own.
I hope that this answer is helpful. If you have more questions that need answering, feel free to post them as separate questions. Be as specific as you can in asking your questions so that you will get the sort of information that you are looking for.