The Elesa of the early English sources has been identified with the Elafius of the Life of St. Germanus. I hold to this identification. As for the former, Kenneth Sisam (supported by David Dumville) attempts to prove that Elesa is a derivative of Aloc/Alusa from the Bernician pedigree.* If so, there is no need to find a Celtic prototype for Elesa/Esla.
Of course, if this is true, then the very early St. Germanus story would have to be dependent on the Anglo-Saxon genealogy that grafted Aloc/Alusa onto the Gewessei line of descent.
*As written, Elafius is a Latin name derived ultimately from Greek elaphos, ‘hind, stag.’ A son of Ceredig son of Cunedda is named Hyddwn, from Welsh hydd, ‘stag, hart.’ He was the grandfather of St. Teilo of the stags. It is possible, then, that Elesa is not from Aloc/Alusa, but is a corruption of Elafius, itself a Latin translation for Hyddwn.