That's a double standard though, J2b2-L283 is not common as well, but that was not the case in ancient times. I think this region was severely depopulated because of the War of the Batos, Illyrian revolt of 6-9 A.D where most of the male population was enslaved and put to work in Italian mines.
You are right that its probably a recent founder effect, but its still remarkable that just across the border to the next cultural formations, the increase is so steep. Yet it might be anachronistic and simply being lost due to Western Celtic groups, Romans, Germanics and Slavs in particular, with recent founder effects.
Vatin is possible as a carrier, but honestly, even if they had it, I think we're talking about something similar like with Sintashta and Yamnaya. They might have been one group once, but more recent founder effects caused the spread. I mean its possible that Vatin-related groups or what people considered as such did contribute to the formation of Pre-Gava, but still the vast majority of E-V13 will come from Channelled Ware. So if such Danubian groups contributed to E-V13, its early, by having an impact on Pre-Gava. And we have not found anything South of the Danube indicated that E1b1b persisted there, especially not in larger numbers.
How Pre-Gava came up is yet another debate, because it seems to have been rather local in the Northern Carpathian zone. Otomani needs to be considered, but also smaller groups I came across about which little is known and researched, sometimes even their mere existence being disputed. But they all lived there, in the same region between Silesia and Western Ukraine, Southern Poland and Northern Hungary-Romania.
Why exactly E-Z5018 should be an earlier branch, being earlier in the Balkans is something I don't understand. There are some other E-V13 branches, especially some of the smaller ones, which could have split earlier. I don't believe they did, but they could have or its more likely for them than for E-Z5018.
One thing is interesting by the way, Scaledinnovation puts most of the main lineages in the same general place for E-V13: http://scaledinnovation.com/gg/snpTracker.html
I think that's because of the earlier mentioned, fairly even spread of the lineages. They are so evenly spread, that even though the region pointed out by the tool is not correct, they all land in the same place. You can do it for all major branches up- and downstream. Doesn't matter. It doesn't tell you where they originated, but rather how evenly spread out they are!
If not, its rather because of later shifts, founding and extinction events. The Western European and Southern European groups pull West and the North Eastern is too weakly sampled, and from areas which being so heavily affected by the migration period, that E-V13 was going down there. But with some fantasy, you can easily put it straight, by balancing that issues out. And that puts you rather North of the Danube and possibly in the Eastern sphere. While a local pick up is possibly, the most likely scenario is E-V13 was already strong in pre-Gava. And looking at where this culture came up, you land definitely North and East of the Middle Danube.