The main argument has been that the great majority of the medieval nobility were fairly new to the scene as late as the eleventh century. Canon law prohibits consanguinity in marriage to the seventh degree, and even in the later period even the most powerful counts and barons could not trace back their family trees that far.

Over the period from the tenth through twelfth centuries, territorial control over western Europe (specifically France) gradually stabilized. Particularly powerful men - essentially warlords - captured territory and built fortifications, slowly expanding their control outward and solidifying a fixed geographic area. This is in contrast to the earlier system of ownership, where land was held by right more than by force, and territories could be widely scattered and separated. Up until the end of the twelfth or early thirteenth century, the ranks of the nobility were fairly fluid, as long as you could muster sufficient power to stabilize your claims.

From the thirteenth century onward, not only is there an ossification of geography by the methods described above, but there is also a population boom, and the amount of uncultivated land in western Europe gradually drops to almost nothing. At the same time in France and England particularly, the central authority of the king is growing substantially, and for the first time we see something develop which we might refer to as a real state in the modern sense developing. This centralization leads to a gradual development of a hierarchy centered around the king, evolving ever so slowly into the rigid structures of the seventeenth and eighteenth centuries.

So, with respect to your specific questions:

how the various nobles fit into their respective societies

This is going to vary greatly based on time, place, and specific nobility.

who were the most important

This, too requires contextualization. Most important to whom?

what the noble's possible responsibilities to each other were

A vassal required his suzerain's consent for any acts concerning his property, although this was originally very loose and only became strict in the thirteenth century. The suzerain in exchange had the responsibility of protecting the vassal. Many other responsibilities might exist, but only in specific contexts.

how laws dictated interaction among highly and lowly ranked nobles

There were formal distinctions of title and address which became more elaborate over time, but I'm not aware of anything particularly notable or universal.

if there was a way to climb up the "nobility ladder" in less than a generation, or if there even was a "nobility ladder" in the first place

I would caution you against using noble titles as a strict gradation of prestige and importance, although this becomes roughly true towards the fifteenth and sixteenth centuries. Effective power and influence do not always map exactly. The general answer to your question is that it became more difficult to effect upward mobility as time went on, and that martial prowess seems to be the surest road for success.

Some sources & reading:

• Southern, R. W. The Making of the Middle Ages. London: Pimlico, 1993.

• Kantorowicz, Ernst Hartwig. The King’s Two Bodies: A Study in Mediaeval Political Theology. Princeton Paperbacks. Princeton, N.J: Princeton University Press, 1997.

• Bisson, Thomas N. The Crisis of the Twelfth Century: Power, Lordship, and the Origins of European Government. Princeton, N.J: Princeton University Press, 2009.