These are two quite different question, as the consolidation of Germanic power into kingdoms and the adoption of feudalism are separate phenomena. I can answer the first part, but I am less qualified to answer the second.
Peter Heather, early in his Empire and Barbarians, relates the works of Roman historian Ammianus, who recorded an Alamanni invasion across the Rhine in 357 AD. The Alamanni army was led by their king, Chnodimarius, and accompanied by his nephew, Serapio. Theses two were kings of greater power than the other Alamanni kings, which is why they led this army that included five other kings and ten princes.
If that sounds like a lot of royalty, it is. Arminius and his Germanic alliance defeated the Romans at Teutoburg Forest in 9 AD, but Arminius was murdered by Germani in 19 AD and the alliance of tribes quickly fell apart. When the Alamanni were defeated by the Romans a great deal of the royalty was either killed or captured. Chnodimarius himself was captured while fleeing the battle. This allowed the caesar, Julian, to impose harsh terms on the Alamanni. In the times of Arminius, these terms along with the staggering defeat would have effectively dissolved any Germanic political cohesion. But by the 300s the Alamanni persisted through this setback, a testament to a political strength that did not exist in previous centuries.
It is difficult to pinpoint when the Germanic tribes transformed into kingdoms because it was a gradual process. The Alamanni of the fourth century are clearly a different breed of cat than Arminius's confederation of the first, but it is difficult to say a lot definitively about them because Ammianus and other Roman historians wrote from the Roman perspective. This is the general truth of accounts for the Germanic peoples: what we know about them mostly comes from Roman written sources or archaeological record. This makes saying anything about them more difficult, especially since Roman sources are finite and they also presumably were thin on written documents explaining the political structure of groups such as the Goths, Franks, or the aforementioned Alamanni.
However, what does seem to be the case is that by being in proximity with Rome, the Germanic groups became increasingly sophisticated. By the 300s their lands were probably being passed down from parents to children or at least kept within family, and trade and raiding across the Rhine was common. This lead to what were essentially Frankish, Saxon, Alamanni, Burgundian, and Gothic client states on the frontier who were assimilating Roman culture.
By the 450s these groups and many others either were migrating or had already migrated into Roman Territory due to a number of factors, not all of which are clear, but the Huns were certainly a significant one. None of them were actively trying to destroy the Roman Empire. On the contrary, they were looking to assimilate into it and further adopt Roman customs. This was only possible because of Rome's waning power. In previous centuries migratory groups would be defeated by Rome, broken up, and scattered across the Empire. But when Rome was unable to defeat the Tervingi Goths after they crossed the Danube into the Balkans in the last decades of the fourth century it became clear that some Germanic groups were now sophisticated and powerful enough to defy Rome on their own soil.
These groups ended up carving out their own kingdoms from Rome, but the kingdoms incorporated existing Roman traditions. From Rome a kingdom could derive legitimacy, as we see throughout the centuries with the Holy Roman Empire, the Sultanate of Rum, and the Third Rome. Even the Saxons, who for a long time were assumed to have annihilated the Romano-Celts of Britain, have been shown in the genetic record to have largely assimilated with the region's previous inhabitants.
So to summarize in an extremely brief manner, proximity to Rome allowed Germanic groups to gradually adopt Roman traditions and blend them with their own. When external factors encouraged Germanic groups to migrate into Roman territory, they were powerful enough to retain their cultures but it was advantageous for them to blend it with the local Roman cultures in the various places they settled to enhance their legitimacy in the eyes of their subjects. This led to Germanic kingdoms in Iberia, Gaul, North Africa, Italy, and Britain.
The Germanic tribes stopped being a tribal society after feudalism emerged. Essentially, this marked a shift in allegiances which had already been underway, though in a very slow process, for centuries.
The main social development among the Germanic peoples during 50 BCE-500 CE is the formation of increasingly larger confederacies; ie. the Franks, Goths, Alamani and Saxons of Late Antiquity, as opposed to the smaller and more autonomous tribes that came before them. Instrumental in this process was the proximity to the Roman Empire.
The Roman Empire, for a long time at least, formed a very powerful and stable political entity. In forming political ties to the local Roman bureaucracy and providing warriors as auxiliaries to the Roman Empire in return of payment, the Germanic elite became stronger and their society (though never egalitarian) became more hierarchical. More simply put, the Roman Empire created an incentive for the Germanic tribal elite, to divide their loyalties between the tribe and Rome; whereas earlier there had only been the tribe.
When Rome collapsed and the Germanic kingdoms fully emerged, the tribe as an all encompassing social entity had already begun to disappear. The Germanic kingdoms were too large to allow it to be effectively ruled by a tribal system. What emerged instead was a mixture between the former local Roman bureaucracy/upper class and the warrior elite of various Germanic tribes. This elite warrior class (proto-nobility) was given lands in order to support themselves and ensure their loyalty; thus beginning the feudal system; in which the village or fief to which a person was bound, not a clan or tribe, formed the center of political life or self-identification.