Right, you seem to be asking a lot of confused questions. At this point it might be better to just take a step back, re-evaluate the sources we have, and look at this things from a more abstract, anthropological perspective rather than a muddy, historical (in the sense of textual) one.

Kin-groups by definition are cultural constructs, of which descent is just one (and usually the most important) factor. Thus there is no contradiction in including both 'family' (as in blood relatives) and others within your kin-group. There can be many levels of kin, from the household to the natio, or people. Tribe is a bit of a useless concept here, falling somewhere in between, and is not really a concept I would use without further clarification. Earlier authors have been too confused by the Victorian notion that 'tribe' for Germanic peoples meant the same thing as for Plains Indians or Central African peoples.

Beyond the level of the kin group, however, there is also another level of community based on class. Particularly higher-class people would feel more connected to other aristocrats from further away than to people from their own kin group. These people in the Germanic period increasingly come to emphasise their separation from the 'regular folk' (plebs if you want) through different material culture, often creating their own peculiar mix of Roman and Germanic (or other foreign) elements in for example architecture or burial.

Of course, some ambitious characters might attempt to intermix those two levels of social organisation, a land-based agricultural conservative one and a martial aristocratic cosmopolitan one, to mobilise larger and more effective power structures. This creates friction and historical events that also emphasise difference between the two modes of organisation, which makes them relevant and confused in historical texts. A common way to do this is for an aristocrat to present himself as protector of a household; he adopts symbolism associated with agricultural practice, for example, or presents himself as protecting the ancestral rights of a specific kin-group. This can also include taking up a specific tribal name, such as the 'Suebi' or the 'Franks'. One should keep in mind, however, that these cultural emblems (names among them) are flexible, and used as tools, rather than pre-existing structures.

Note also that in certain areas and at certain times, either mode of organisation (village or aristocratic) could be more dominant than the other.

Beowulf, because he is part of an aristocratic environment, does his 'internship' or 'cultural exchange' with a foreign group, yet when he later establishes his own power structure he makes use of both his ancestral kin affiliation and his foreign contacts. This way of manipulating ethnicity might not have been available to the regular Jutlandic hillbilly, who would rather fight to defend his own rights to inheritance within his village. He might also decide to support a leader who promises to defend these ancestral village rights, and thus still end up in a foreign retinue. Flaunting foreign-ness might however be less acceptable for him than it is for Beowulf, unless he wants to relinquish his ancestral rights.

To what extent serving in a militia (or a retinue) was an obligation and to what extent it might have been voluntary is still a very open question, and this probably varied depending on time and location and/or charisma and ambition of the strongman involved.