Some suggestions on 1. When historically Slavs came to settle to the Balkans? From where they came?
I have studied a chapter Освоение славянами Балканского полуострова и Пелопоннеса in В. В. Седов. СЛАВЯНЕ Историко-археологическое исследование. (Освоение славянами балканского полуострова и пелопоннеса. Валентин Седов. Славяне. Историко-археологическое исследование. Книги по истории онлайн. Электронная библиотека). I hope to reread this chapter in authentic copy, and may look through other chapters.
Это одна из монографий на тему славян.
So the brief summary is that the movement was from Middle and Lower Danube in 6-7th centuries (as they were spreading all accross that area). Very often as military raids, sometimes in coordination with Turkic tribes (авары) against the Byzantines (weakened by wars with Persia in this period). Settlers/colonists (mostly agriculturist Slavs) would then move to lands secured in miliary raids. It doesn't mean all colonisation was warlike though. The colonisation was in different ways and at different times. Basically it's spread accross 6-9th centuries.
Now it is known that assimilition in different directions is taking place all accross the Balkans (Slavs mix with Byzantines and are romanised, or the local peoples in other places are assimilated into Slavic communities). So as a result modern Slavic peoples (former Yugoslavia, and Bulgaria with its Islamic influence) can be viewed as the descendants of Slavs that came to dominate the respective regions. On the other hand, assimilation of Slavs takes place in Greece (Slavic presence in Greece in 7-8th centuries) where Slavs are completely assimilated into the Greek culture (Slavs in what is today Eastern Romania seem to mix with the local peoples and take part in Romanian ethnogenesis, though this matter is contraversial according to most Romanian scholars; Slavs in Albania are also assimilated).
The chapter in that book is fascinating, looking forward to dealing with it again.