E-V13 is existent in the old Illyrian regions, closer to the core the more percentage there is, nonetheless it does exist among Croats as well as among all other southern Slavs (not only southern even).
But that can’t be since Haplogroup I is noted in Balkans much before arrival of R1a stock, it means they didn’t come together which would again indicate they we’rent one tribe of people.
Anyway these are the flaws in your theory:
1) If Paleo-Balkanians were E then why did Greeks view Illyrians as different people both in looks and language.
2) If Slavs were both R1a and I, that would mean Dalmatia (which was very populated) was empty before Slavic arrival even thought we know Slavs didn’t settle the cities right away rather only started to merge with Dalmatians as a very slow process. Last Dalmatian dialect speaker died in 1898.
3) Only 6% of Croatian Haplogroup pool is made of E-V13, that would be impossible to be the only trace of native Dalmatian population.
4) If Slavs from the begging were I and R1a hybrid, intact Slavic populations of central Belarus would have far higher amount of it. Hp I2 is non-existant in all Slavic countries except southern Slavs (which migrated to a land that already had it’s population, it would be logical they have their imprint then no?) and Ukraine which is in close proximity with South Slavs and yet there is also very low, just 12%.
5) If E and J are “southern-European”, and I and R1a are Slavic where is the genetic imprint brought to Balkans by Turks, Turkic Bulgars and neolithic farmers as Berbers?
6) If E-V13 is again southern-European as you say and was among Romanised paleo-Balkanians as Vlach then why is it’s amount in Romania only 6% of it’s whole genetic pool and yet they are the biggest remnants of Vlach population in Balkans and carriers of Romance language while it has much bigger I2 percentage (18%)?
7) Again why would E-V13 then leave the largest imprint among Albanians than among Greeks and almost non-existent among old Dalmatians even thought last two were few dozen times larger populations than off Albanians and their ancestors.
In generally your theory doesn’t provide a good reason why would only South Slavs have a I2a imprint while West and East Slavs don’t while we know through history it was only the South-Slavs that were migrating and settling already a inhabited land. I’m sorry man, it’s too flawed.