Rather than looking at historical 'tribal structures' of the peoples you mentioned, it would make more sense to look at traditional village structure in Northern Europe from a more distanced perspective. The basic principles of land use rights and village inheritance law had hardly changed since the Iron Age up to the Enclosures of the late 18th century in Britain (and the creation of modern economies elsewhere). Traditional concepts like shared land managed collectively by the village, and inheritance based on ancestral claims, dictate lots of petty politics in the majority of society. In such a 'primitive democracy', exact spheres of influence are sharply defined by custom, and together with a system of inheritance that divides up the land rights of the parents among the children this prevents the accumulation of power in one individual. These anti-authoritarian (on the surface, and in theory, of course) communities can still be observed in the more rural parts of Northern Europe.

It is this traditional system of land division that is repeatedly attacked by outside forces (and has to adapt to historical circumstances) over history. When William invaded England, for example, he had to make concessions to the representatives of this local power system in some areas (like Kent), but used his military power to break down the system and impose his own in other areas (like Northumbria). Similarly, while during the Medieval period the aristocracy and the church was largely content with levying taxes and leases from a largely independent village structure, it had to resort to using the force of the new nation-state to break down this ownership structure in the Early Modern period. In this context, traditional power structures become attractive for American revolutionaries interested in primitive autarky, as the struggle between aristocracy and peasantry was still very much ongoing in this same period.

This is an interesting question.

Most of what we know about Germanic 'democracy' comes from a Roman historian, Tacitus, who describes them as egalitarian, freedom-loving, etc. Historians accepted Tacitus' descriptions as accurate for a long time, but recently have begun to question the text. The current consensus is that Tacitus was actually not particularly interested in how German society was structured, but was instead making a point about how degenarate his own ROMAN society was by saying, 'look at the barbarians, they have no civilization, but even THEY can figure out how to be free and moral and democratic. Why can't WE?' This means that many of the 'democratic' virtues ascribed to Germanic people were actually pieces of Roman, not Germanic, society that Tacitus ascribed to the barbarians for rhetorical effect. (See Krebs, A Most Dangerous Book, for more on the fascinating history of how Tacitus has been misunderstood for 400+ years).

The early Anglo-Saxon laws, which survive only from the late 6th or early 7th century, are very different from Roman laws, but they're mostly a set of fines the guilty party must pay for murdering or injuring someone. Institutions like the witan emerge later, and it's difficult to connect them to ancient anglo-saxon traditions, because most of what historians thought they knew about those traditons came from tacitus (who, like i said, was actually talking about roman society and projecting it onto the germanic tribes).

The big issue here is that, while kinds of democracy show up at different points at different times in history, those points don't always evolve out of deep traditions. Rather, they emerge from long chains of events which might not have much to do with democratic traditions to begin with. In England, the monarchy's need for tax revenue to fight the hundred years' war meant the king had to tax other powerful, important people, and that required their consent (via parliament), since the king was too weak to force them to pay. That eventually became the democratic system england has today, as other important groups were given representation to encourage their cooperation.

So where do the Greeks and Romans come in? Ideas (like democracy) can do two things, often at the same time. They can justify actions and events that are already happening (like when the american revolutionaries called George III a tyrant, evoking the shared memory - for everyone who had read Livy- of how the romans overthrew their tyrants and set up a republic, in order to give their rebellion more legitimacy). But they can also shape the direction those events take (like when polybius' description of roman government was used as a model for the american system). Usually both processes happen together: people reach for things from the past to justify their actions in the present, and these things from the past influence and shape people's actions, even if there was no direct evolutionary link between the two events. The fact that the founders read the classics, and looked to the romans for inspiration, was enough for them to model a society after their ideas (and people accepted it because they, too, probably knew a lot more about the roman and greek systems than they did about the anglo-saxons).

The interest in anglo-saxon law and society only really took off in the 19th century. The chief reason was that few scholars had studied the old english texts before the 19th century; but that changed in the 19th c, as a result of the growing interest across europe in national origin stories. By the end of the 19th century, hundreds of thousands of histories of anglo-saxon england had been sold to english school children, and americans like henry adams were writing eagerly about america's supposed anglo-saxon legal heritage.

But american democracy didn't evolve directly from greece/rome or the anglo-saxons. Rather, it emerged from the historical circumstances that led up to the revolution in england's late medieval and early modern periods, and used the older 'democratic' systems as a source of inspiration and legitimation for the new government. It used a system that educated people understood to create something new.