Bastarnae aside, what does "Galicia" remind you of? Celtic Gauls. And supposedly that is the core region of White Croats if i have understood correctly, because there are a number of maps on the internet. Do read the history of the region in Wikipedia, where it describes Celto-Germanic and Celtic proper presence,
You go too wide. For now, we barely know that mutation I-S17250 is from south Poland. It is connected to Croatian ancestors. And who earlier passed there, stayed, etc is even harder to guess.
Celts have influence and in that area, they also had an influence on the territory of today's Slovenia and Croatia. But very little is seen in Croatian genetics.
Bastarnae aren't mentioned though. I believe it is evident that the region had a Celtic presence.
For now Croats don't have much
Celts genetic, do they have any Germanic haplotypes possible, but and this haplotypes and branches are in small percentages.
Your mistake is that you only treat as Celtic the populations of the British Isles, while it is known that most Celts were eventually assimilated and prior to that encompassed vast regions of Europe (east, central and west).
Celts are Indo-Europeans, I2a people are not Indo-Europeans. Was there any interference between these groups, possible. But you are going too far into the past. Let's say that Celts go to Britain, part of the Celts remain and they mix with I2a peoples. Again, this interference must be seen in genetics, unfortunately it can not be seen.
eupedia
Alternatively, I2-L621 lineages could have lived in relative isolation from the mainstream Proto-Indo-European society somewhere around Ukraine, Poland or Belarus, then as the centuries and millennia passed, would have blended with the predominantly R1a populations around them.
If they are assimilated by the Celts, then Celtic European R1b genetics was also isolated together with
I2-L621 lineages. And R1b genetics also must be developed after Slavic assimilations. But as we know this is not the case. Croats have 6-7% of R1b, part is probably of Balkan origin and part of Western European origin. However, I don't know percentage of R1b which would come to Balkans with the White Croats. Let it be 1-2%. in Croatians. It's actually nothing, and since they (White Croats) are close to the Germanic tribes it is probably from them.
However, it is possible that Croats do not have any R1b which coming with them from White Croatia.
That quote from Eupedia seems outdated, because in YFull there are a number of I-L621 individuals negative for I-L147.2 (CTS10228) in central and western Europe, https://www.yfull.com/tree/I-L621/, seemingly most. Don't know if there are any additional in FTDNA.
This means that older mutations have a source
in central and western Europe and probable from there someone coming to south Poland (5,000 ybp)
I know that I2a is from the Palaeolithic in Europe (it formed there after all), and that I-CTS10228 was also obviously assimilated by Indo-Europeans such as the Celts. As for R1b haplogroups, again consider the founder-effect phenomenon of I-Y3120, as well as R-S28 (present) and R-L21 (absent or very minuscule) mentioned above.
You go too far into the past, if there is assimilation then it must be seen in genetics. Assimilation or mixing with the Celts is not seen. If you see it, show me genetic data which prove this. Then I'll answer you.
There is no genetic evidence for Bastarnae bringing I-Y3120 or I-Y18331 since we lack ancient relative samples, yes, but it is suggestive and it is the scenario that makes the most sense, along with Celts proper if we account for an earlier TMRCA.
Unfortunately
I-Y3120 or I-Y18331 has nothing to do
with Celts , nor does current genetics prove this. Regarding
Bastarnaes, whether they bring
I-Y18331 or not to Balkans future will tell. What we know for now is South Poland as a source, later there exist White Croatia and probably from there begin Slavic migrations to the Balkans.
I-Y3120 is characteristic of one ancestor from the 3rd-2nd centuries BCE somewhere in southern Poland, yes, it seems so. But he could have descendants in Poland and Greece at the same time if we are to consider the Celtic proper origin of I-Y18331 in Greece and a TMRCA of the 3rd century BCE (like it used to be some years ago in YFull). He could also have descendants in the Carpathian region and the southern Balkans or Greece if we are to consider the Bastarnae origin of I-Y18331 in Greece and a TMRCA of the 2nd century BCE (like it is currently in YFull). Both of those two centuries which are associated with the TMRCA of I-Y3120 include migrations of Celtic populations to the southern Balkans and Greece as well, therefore there is nothing out of place in this scenario.
Do you understand that Celts have their own genetics and it is R1b.
I-Y18331 mutation went somewhere, and where we don't know yet. We will know this (based on living genetics) in a couple of years. Migrations up and down make no sense.
You write, "but it can also mean that people with this mutation in Greece explore their genetics better.". I highly doubt that mate. I am actually one of the few interested in the matter here in Greece. Even many Greeks who have tested themselves are from the Greek diaspora. This is probably due to lack of interest and knowledge in the matter along with the high prices of the tests, considering the average wage of approximately 650 EUR (judging from experience) per month after the crisis, ironically higher than Ukraine. Furthermore, the Ukrainian DNA project in FTDNA lists 1861 members, while the Greek one 724 members.
Return of someone from Greece to Belarus? Therefore in the future it will be shown that Ukraine is a probable source of this mutation.
Archaeogenetic data would indeed be more ideal and hopefully we will see Bastarnae and eastern Celts being examined sometime soon, i hope.
We will see.