Thanks. I know all that, lol. I read Oppenheimer's book a couple years ago along with Bryan Syke's book Saxons, Vikings And Celts.
IMO, Oppenheimer's book is much better than Sykes' book.
BTW, 52% of the Skolt Sami are haplogroup 'V'....other Sami groups are U5b.
Being the knowledgeable fellow you are, I thought you must.
Syke's book is sitting on my desk and is next on my reading list.
For the last couple of years I've been focusing on Indo-European languages, their relationships, and what they can tell us about migrations/relations. I'm just now getting into genetics. As Oppenheimer and others have pointed out, that can be more of a cultural shift rather than a population migration, e.g. the Frisian/English relationship (by the way, I find that it is actually easier to read old Frisian than Old English).
I am wondering, however, how much of the genetic patterns are affected by particular migrations and how much might be affected by disease resistance in particular groups. I'm sure over 50k years you can have several pandemics that can wipe out a population, or nearly so. Perhaps some of these groups have a slightly higher resistance than others and that can lead to some unevenness in the distributions.
Another interesting point to me that Oppenheimer makes is how hard it is to actually replace a population once it is established. To fundamentally change a population you would need a pandemic or genocide followed by another mass migration, not simply a Viking or Saxon incursion.