I have looked at the data, but have had difficulty coming to any conclusion other than this: If there is any non-Germanic I1, it has become non-Germanic only recently. Let's look first at the distribution and dating of the major I1 subclades:
I1f: ~2000 years old with a particularly English distribution, although with some membership in Scotland, Ireland, Germany, Denmark, and even Poland.
I1d*: ~3000 years old with a very Nordic center of diversity but with a distribution everywhere, from Scandinavia to Turkey to Russia to the UK to Italy.
I1d1: ~3000 years old with a more southern center of diversity than I1d*, but still an obviously Germanic distribution, with a lot of membership in the UK, and interesting membership in Croatia.
I1d3: ~2000 years old with primary membership dominantly in Finland. I would suggest it to be Uralic if it wasn't for its obviously diverse (just less frequent) presence in the North Germanic countries, and its youth. As is, it looks like an expansion from Scandinavia on Finland.
So what of those spikes in interesting places? Not all I1 fits into subclades. Since Nordtvedt's STR clusters are the gold standard of I1 STR clusters, let's analyze them in the context of those:
The spike in the Balkans: This one is the least researched due to the lack of samples from the region, but so far, the most common cluster for Balkans members seems to be T2. T2 is an odd cluster that stretches from Ireland to the Balkans and seems to have common membership everywhere but at its center of diversity, which could indicate a displacement of I1 from its center of diversity (maybe close to Poland?). But although it's somewhat mysterious, T2's distribution is clearly not Slavic or Illyrian, and everything about it indicates Germanic, possibly a combination of East, North, and West Germanic, with its appearance in the Balkans apparently East Germanic. Other clusters present in the Balkans include different AS clusters, which are also apparently Germanic. So barring us finding an interesting Balkans-exclusive cluster, it appears that the spike (and I should probably use "spike" loosely because it really isn't all that common) in the Balkans is also Germanic.
Altogether, I can't find any I1 that can't be explained by a Germanic migration. Can you?