I see a lot of confusion here.
Know this! 10th century is a whole different world from the 7th century, which is a totally different world from the 5th century.
Most people ascribe a modern mentality to these ancient people. They weren't nationalistic, they didn't fight to preserve their language,
they just used whatever they thought was beneficial at the time. Multi-ethnic tribal confederacies formed and dissolved, and the warrior elites were never a majority,
the majority was ordinary peasants, but only the elites left a trace for historians.
So, today some historians and most laymen ( wrongly ) tend to think that ancient states had firm and defined borders and identities, and that the usual ethnic composition was just like today
( today, people live in countries where the ethnic majority usually defines the identity, name and language of the entire population - back in ancient times, an alien tribe of 10.000 warriors could subdue 500.000 peasants,
and form the nobility of that mass of people, but from then on there was no blueprint on what would happen next.
There were no modern militaries occupying countries ( like Germans in Poland, or USA in Iraq ), back then, the people couldn't care less if they were ruled over by Avars or Goths, just as long as the taxes weren't too heavy,
thus there was little resistance from the common folk to new invaders.
When Ostrogoths "conquered" Italy, they weren't like the wehrmacht - they were more akin to a military hunta who gradually won political power within a state.
When Slavs came to the Balkans, they weren't like a modern invading force rolling with tanks and razing villages. The locals didn't define themselves as Illyrian or Thracian - the locals at the time probably were fully integrated into an Eastern Roman Empire style of life - they probably used Latin and Greek language and names and surnames, and probably some still used Dardanian, or whatever at home.
Ancient Empires were more like USA ( a melting pot ) rather than ethno-nationalists like Nazi Germany. The "barbarian" tribes weren't much different in that aspect. They had a tribal identity, but knew that they have to be flexible rulers ( just like their Roman or Persian or Chinese imperial role models ).
There's also a lot of confusion around haplogroups. Early medieval people were already mixed - for tens of thousands of years already!
All of the haplogroups came from Africa across Asia, and into Europe. If you roll back through pre-history, you'd probably discover that R1a/R1b or their ancestor "P" had some proto-mongolian features, and/or blonde and red hair unlike almost anything we can see today... and then many generations later, in 4000 BCE they entered Europe to find Europe was already populated by J and E, and G, and I etc.
But was it their first time they came across individuals belonging to other haplogroups ? No! These earlier populations in Europe also moved from Europe into Asia, or back into Africa.
The only borders that defined these ancient times were natural obstacles, like the Sahara, the seas, the mountains. That's the only reason why you don't get to see ( more modern ) sub-saharan African DNA in European populations, just like there's little European genetic influence southeast of the Himalayas.
