I forget which book (perhaps TC), but Ebenezer tells Harry that the ancient notebooks in his study at Edinburgh contained the thoughts & writings of Master wizards stretching back to Merlin, implying that at least Eb’s magical lineage (not necessarily genetic) descends from the original Merlin.
Harry, having been apprenticed to Eb for a few years would also have learned magic in the style of the original Merlin (Harry uses Latin, which the OG Merlin made the official language of the White Council). Harry also seems to have a vague degree of familiarity with the magic Merlin used to create Demonreach when Bob plays him the film reel of its creation in CD.
FWIW, Eb also tells Harry that Merlin learnt magic from Odin (which explains why Odin is so familiar with the banefire at Demonreach in CD).
Regarding Mab - I think it means that she was like Sarissa - a Changeling who eventually chose her Fae side, but had spent significant time as a mortal. Given the cluebats about the relationship between Hecate and the Fae Courts, I would suspect Mab’s lineage would eventually go back to Hecate.
i think that was turn coat, yeah that was also one of the factor why i think harry actually is blood related to original merlin, that their mentors were actually generations of family and the journals only passed to theirs.
on why harry got familiar feeling, that's probably a good guess, although i suspect it might have been the echo of his future, similar to the disturbance from the island days before the actual time those outsider & the ladies attacked.
yeah, most probably mab was a changeling, and probably her mortal parent was a significant figure. i sort of suspect that most if not all of faes are related to mother winter & summer myself, that it's more like the vampire & their spawns relationship, only mortals were also mixed in there. and the ones who are fit to be queens are women who have royal bloods, and the higher their station is, the more significant the lineage needs to be.
