1. Until fairly recently the dominant theory was that the Picts were non-IE and therefore the language
2. The union of Scotland by Kenneth MacAlpin is commonly held to be due to his satisfying both Pictish and Gaelic rules of succession
3. Working from a Scottish document now in France it is suggested the union wasn't completed until a lot later, that after Cinaed's death the legitimate successor was murdered and a usurper removed all other potential kings apart from his own lineage (though of course as last of the mormaers of Moray Lulach the Fool, son of MacBeath, was the last legitimate Pictish contender much much later)