Hi Andrea,
Most of the Manx C, Q and K surnames were formed during this period and your info on Cretney fits the general position. Although all such analysis based on the possible original meaning of names is conjecture, the origin of Cretney from 'son of the Briton' is as good as any and suggests an early integration by a Brythonic Celt from Wales, Cumbria or Southern Scotland into an Irish/Scottish Gaelic community, possibly following the Dalriadic colonisation of south-west Scotland which pushed the Pictish culture north and eastwards and the Britons southward. It is likely that the paternal ancestors of the Cretneys came from Galloway or Dumphries before Manx records of individuals became widespread - and the original ancestor may have moved to the Island at any time before 1500, although it may be most likely during the period of Scottish rule after 1313.
DNA testing, if you went down this route, may show ancestral links to some of the longer established families in Dumphries and Galloway and possibly further back to Northern England, but this would require you to establish contact with likely Scottish McBretneys etc. If you try this, many readers on this website would be interested to know the result.
Ian