I never heard of a 'rift', just this letter dated Dec 11, 1916 written from Zenaida in the Crimea to Felix in St. Petersburg:
in part:
I do not like the celebrated "sister" [the Grand Duchess Elizaveta Feodorovna Romanov]. In my opinion, she poses most impudently as a member of the family. She calls Irina by her Christian name, and evcn "Tata"! I strongly object to her behaviour. When I visit Irina I constantly find her there, lounging in a chair, without in the least understanding that her presence does not interest me. She sports a kind of operatic costume of a Sister of Mercy, and generally spoils the whole atmosphere by her presence. Irina seems to accept it all as inevitable, whereas I simply cannot stand it. You will see for yourself soon, when you come home. In the meantime,
I send you my best love,
MAMMA
This shows me that contrary to everything I've read before, including Felix's book, Ella and Zenaida were not always on the best of terms. It's my guess that Zenaida was a bit annoyed with Ella's 'sister' act and wanted the 'old' or 'fun' Ella back. I doubt Zenaida had the nerve to say this to Ella's face, and if anything else happened between them I don't know of it. But this does show another side of things. They were still united in the anti-Rasputin movement and I assume were at least somewhat friends to the end.
Still hoping for pics of them together! Brian?