We don’t posken by genotyping.
Why not? It’s a question of metzius, and this is important evidence. But you have it backwards. If they had genetic markers of a Jewish past, then there would be reason to take their claims seriously. Without it what reason is there?
All the reasons you list are why they need geirus. The fact that they claim to be Jewish and have been for as many generations as they can recall is what makes it a safek.
Are you serious?! Just because someone claims something creates a safek that they might be correct?! If I claim to be a lamp post, will you entertain even the slightest thought that maybe, just maybe, I might be one?