Some LIs can be convinced to switch sides, but Alpheon is not one of them--he is the most restrictive of the LIs, and that is by design.
If you want to have a successful romance with Alpheon, going back to the conversation with the Prodigal One and initiating the LoD ending is the only way to do so (Alpheon locks you out of the M ending and will effectively mind-control you into not romancing him in the DH ending). As I said, this is by design, as Alpheon truly loathes what he has to do in the DH ending, and MC ignores his wishes and forces him to act. That's what prompts his 'rebellion' in the caves--he gets a taste of his future.
(The short story 'The day I decided to cry' deals with his feelings on the matter, although the protagonist there is implied to have romanced Malec instead. In a successful DH ending, Alpheon spirals into suicide ideation.)
There is a paid guidebook DLC available on itchio and Steam with a guide to choices, branches, achievements and romances!