I found an alternative opinion about who provides the model for Elstir:
Paul César Helleu 1859-1927

Helleu ... moved among literary circles and became great friends with Marcel Proust who based the character of the painter Elstir in "A la recherche du temps perdu" on his friend. He requested Helleu do his portrait on his deathbed; and in 1922 Helleu obliged with a profile done in drypoint.

From a blog, source cited as Kay Lopata Fine Art web site (kaylopatafineart.com)