I don't understand the proposed scenario. A population that was J dominant introduced I2a to the Balkans, when?
An Adriatic origin of I2a is within reason, although I'm inclined to disagree. It also depends somewhat on what time period we're discussing. If we're discussing undifferentiated I2a way back in the Paleolithic, then sure, guessing the Adriatic is as good as any. I've guessed I2a as Franco-Iberian during that time period, and I have at least as much support for that. But it's more difficult to maintain I2a as Adriatic once it differentiates. None of its branches--M26 "Sardinian," Dinaric, Western, French outliers, etc.--have their highest modern diversity along the Adriatic. The only I2 group that I'd guess to be anchored solidly to the Adriatic is I2b-ADR, maybe complimented by its more successful closest cousin, I2c, but those are both rare in comparison to I2a.