The Breton name Alan is well-established in both Brittany (900 on) and England (1100-1400). There is no record of a name Alan or Alun being given to a Welshman prior to 1100. It must be assumed that the river name Alun represents an early personal name, not recorded as a personal name. But it is the Breton Alan that we find in Wales after 1100, and it is this name that becomes the modern Alun, probably influenced by (but not deriving from) the river name. And yes, Breton settlers were originally the from the same people that later became 'Welsh' and 'Cornish', but obviously the resettlement of Brittany happened before the separation of the three languages.