That's a lot: it yields ot for one but for 3 essays. And it also gives room for many opinions.
I really don't know where to start, and obviously I would have to
consult materials on items I'm less knowledgeable like Justinian or
Theodoric.
If you have time, I suggest you to open three separate topics for each
of the questions in this forum and let the brainstorm work alone...
I will just limit my apporattion to the 1st question (and will exclude Theodoric by the moment):
Charlemagne's state is archetypical of the Upper Middle Ages. Like
other Germanic kingdoms the Frankish realm was born as forederati
(allied) of the Empire and was its "legitimate" successor. Unlike other
Germanic states, Franks accepted Catholicism and didn't attempt to keep
an ethnic apartheid, what probably helped them to prosper in the long
run.
Roman Empire was already feudal and Christian at its fall, German
invaders only took over the administration, introducing little if any
Germanic custom or law. Probably the Germanic tradition is the less
important of the three mentioned.
The Roman one though was essential: Western Europeans felt Romans
(Latins) at that time (with some exceptions like Basques surely), spoke
Vulgar Latin and were used to Roman law. Romans had also introduced
already most of the feudal and Christian elements that would shape the
Middle Ages, though these "innovations" had found some resistences
(Bagaudae mostly) and probably helped to percieve German invaders as a
lesser evil too.
When the Western Empire collapsed, only the foederati and the Catholic
Church stood as institutions. Literacy became secluded into monasteries
and the Church was the only diplomatic and adminitrative infrastructure
of any relevance.
So we have:
- A Germanic tradition with very limited influence
- A Roman culture that had self-destroyed itself largely in the process of Christianization and feudalization
- A Catholic Church that was the only standing element of cohesion
(though there were dissidents too, like the Arian Goths and Bebers)
No wonder that whatever was created after a period of transition was
largely articulated by the Church: it was the Empire of Charlemagne,
later called Holy Roman Empire.