For Greeks its hard to understand others history let alone Albanian history. I recommend you one famous Austrian historian named Fallmerayer. It will help you understand who the Greeks and Albanians are. Wikipedia is not a reliable source to refer to. Even Greeks write there. Imagine now the accuracy of Wikipedia.
As to double headed eagle of Albanian flag: Yes it originates in Roman times and it was adapted latter by the Church of Eastern Rites.
Understand history? You're the one with the problem. You cite an Austrian historian with a political agenda with racist motivations as a yardstick. His theory was nothing more than political writings aimed to seed hatred and since when do politicians talk straight? Most of them would even lie crooked in their coffin. And then you talk about comparing the Albanians with the Greeks, this is chalk and cheese.
The Albanians were a group of tribes living on the lands of Illyria. There were Celtic, Roman and Greek cities there and they were influenced by Hellenism from what I read but only credited with being able to make mead, wine from honey (and by fermentation no doubt, not by a blessing!). The Greeks gave the world literature, science, religion, philosophy, mathematics and so much more. Where's the comparison?
Fallmerayer's thesis appeared immediately with Greek independence and was aimed to cause hatred of Greece. He wanted to claim that Greece was not part of Europe and really Middle-Eastern in nature. His claim that "not the slightest drop of undiluted Hellenic blood flows in the veins of the Christian population of present-day Greece." And this hatred is there today from the Germanic section. They have done everything in their power to disadvantage Greece, especially with repeated refinancing of the loans and thus blowing them out of all proportions and then calling the Greeks names. Greece is the back gate of Europe. They need Greece secure and strong and able to cope with the flood of migration. Instead of that they are kicking their gate to bits. There is no puting the gate further up because further up open up bigger problems.
On what grounds do a people get defined by some "genetic purity"? All people interact with others and mix to some extent or other. There were influences in Ancient Greece from all the surrounding areas, from North Africa, especially Egypt right around the Mediterranean coast. There are battles fought and no matter which side wins there is intermarriage and genetic mixing.
I had some Pakistani friends in Sydney, one of whom had red hair. He told me that he was a decedent of Greeks that had conquered his area from the time of Alexander the Great. Does that make him less Pakistani. No! Every nation and every tribe is continually changing.
So to be Greek does not mean you have to be a some fantastical Hellenic prodigy that he fantasizes. This is the same mentality that I hear from some people in Australia. If you have one drop of Aboriginal blood then you are Aboriginal, not English or Irish or Italian or Greek or anything else "with a bit of Aboriginal blood". Absolutely not, you're Aboriginal. This is racism. And the converse is true too. There are many who have maybe even half non-Aboriginal blood but are still very much Aboriginal.
And even without mixing there is also genetic variations that take place so that the outer appearance may change enormously over the centuries.
However there is evidence that the Greeks are not over-run or diluted by Slavs or anyone else. There are small communities of Slav and Albanians and others in Greece and they have still a unique identity. They haven't all become one big mixing pot.
As for the ruling dynasty, that doesn't make a people some particular nationality. What you are trying to say is that because the early rulers of the Byzantine Empire were Romans then they were really all Romans. The British have a German dynasty or ruling class that doesn't change them to German. The British Raj in India didn't wipe out the Indian identity.
What Fallmerayer doesn't take into account is that the identity is not tied to the make up of the body. There is a lot of this being claimed in the biomedical sciences where the claim is that the body is a machine, that the mind is born out of the complexity of the brain and that is it.. you're a machine.. a robot. The reality however is very different. There are mental and spiritual planes that are not physical, and which play a bigger role in who a person is, and the identity of a race of people. The Intellectual brilliance, innate harmony and simplicity are not qualities of a machine.