"In this case, 'Alun' (early) is a river name from a personal name."
This may show that I'm not educated to a PhD standard, but are you saying that Alun (early) was a personal name?
"'Alun' (late) is an adaptation of the name Alan, from the Breton 'Alan', influenced (possibly) by the river name Alun, but not deriving from it (sorry)." Why influenced (possibly)?
"Perhaps you'd like to explain how this is relevant?" : Breton evolved from Brythoneg(early Welsh), along with modern Welsh and Cornish, so how is it not feasable that the name Alun changed to Alan when the Britons/Welsh first moved to Brittany? From my knowlege of Brezhoneg (Breton), a lot of the words and names vary (from Welsh/Brythoneg) only with different letters 'a' changes to 'e', 'u' or 'y' or vice versa.