It's difficult to follow the "Picts never existed" line.

I'd heard (from the "Picts existed" school) that some of them may have migrated to the north of Ireland where they were known as the Cruidhne (which some say is a Q Celtic cognate of Picti, but it doesn't quite fit - there're consonants in wrong places) who merge with the Scotti (or some of them) and migrated back to the Earra Gaidheal (Gael Coast, or Argyll) as the Sons of Erc (i.e.Dal Riada).

There's a claim that they might have spoken a P Celtic language like that spoken by the Brythons of Strathclyde, based on place names and some proper names (although they seem to have had the initial combination "Uu..." where the Strathclyde "Welsh" had Gw (modern Welsh orthography)).

There's also a story that they had an origin myth that they came to the east of Scotland from across the North Sea - which places them in the southern Norway-Jutland area - which is where Strabo/Ptolemy/Diodorus Siculus placed the "Cimbri". I think I read all this in Kenneth Bannerman's monograph on Dal Riada, but on reflection it's likely that it may have been from various places.