I think there is growing consensus that modern I1 has higher diversity in Northern Germany than in Scandinavia, and perhaps higher diversity yet in Pomerania than in Northern Germany. My best guess is that it was a small clade somewhere around the South Baltic, and got swept up in a westward expanding population of some sort (Corded Ware or something).
That's a possibility. It's just a bit hard to imagine how I1 remained isolated from the numerous LBK-related Neolithic cultures in Germany and Poland for nearly three millennia (c. 5500 to 2800 BCE), then that the R1a expansion of the Corded Ware culture picked up almost exclusively I1 and I2a2 lineages with them to Scandinavia, while the majority of Neolithic lineages in the region must have been Near Eastern (E, G, J, T). Even if I1 and I2a2 did maintain their hunter-gathering lifestyle, living in completely secluded communities side-by-side with the Neolithic farmers, how could I1 lineages have become so numerous in comparison to all other haplogroups, even exceeding the R1a of the Chalcolithic invaders at a ratio of 2:1 ? It doesn't make sense.
If the population of northern Germany and Pomerania had been swept up in the expansion of the Corded Ware culture, we would expect a population in which R1a is dominant, and the rest being a blend of various Mesolithic (I1, I2) and Neolithic lineages (E, G, J, T), but with surely a larger Neolithic faction since farmers could keep much larger populations than hunter-gatherers. I can think of three other possibilities, but all pretty far-fetched:
1) I1 hunter-gatherers of northern Germany adopted agriculture on their own by copying their LBK neighbours, or absorbing a tiny number of LBK lineages. I1 farmers then spread agriculture to Scandinavia, where hunter-gatherers belonged to I2a1 (Motala). Since the autosomal DNA of Gök4, a Neolithic farmer from Sweden, was clearly Near Eastern, it renders the possibility of I1 introducing agriculture extremely unlikely.
2) I1 was part of the original Corded Ware people alongside R1a, or was picked up very early on around Belarus and eastern Poland. Through a dramatic founder effect, the Corded Ware people who settled in Scandinavia were mostly I1, with a R1a and I2a2 minority. However I1 has never shown up in ancient sites in Eastern Europe, nor in the Corded Ware samples from Eulau, which contained only R1a. The likelihood is therefore also low.
3) Corded Ware was truly an R1a-dominated expansion. LBK lineages dominated most of Germany and Poland, except in the northern coastal areas, which were almost exclusively inhabited by I1 hunter-gatherers. When R1a arrived along the Baltic coast, I1 people quickly adopted their ways of life and technologies, and the Corded Ware was spread from northern Germany and Denmark to Sweden and Norway by I1 and R1a people. There is no evidence supporting such a far-fetched scenario either.
That is why I think that the most likely scenario is that I1 was the lineage of Mesolithic Scandinavians all along, and that the I2* and I2a1b samples from Motala are simply unrepresentative. Two other Motala samples (#2 and #9) were described as I*, but could very well have been pre-I1.
No proof of that yet though. These results actually fit nicely in the Nordtvedt tree, and in fact seem to help fill in the apparent gap in I2a1b's modern distribution, which has always been split oddly between the far northwest of Europe and Eastern Europe.
I might have had some older version of Nordtvedt tree in mind. It's true that the current tree fits.
I could understand a variable average generation time, and perhaps even climate-influenced mutation rates (although I don't know of any evidence for that...), but if true, how would that apply to I2a-Din, which had a relatively southern expansion some time after the adoption of farming? It would only apply to ancestral I2a1b, and I don't think the exact branching time of I2a1b with I2a1-L1294 has many implications with respect to the spread of I2a-Din, or of our finding I2a1b* in Mesolithic samples.
If I2a-Din had a relatively southern expansion some time after the adoption of farming, then it makes more sense that I2a-Din spread relatively early, first with the Cucuteni-Tripillian culture (4800-3000 BCE), then with the Proto-Thracians of the Multi-cordoned ware culture (2200-1800 BCE), and eventually with the Daco-Thracians and Illyrians, rather than with the Slavs several millennia later. Wouldn't it ?