I am pretty sure that Eleanor (& variants) and Elena are completely different names .
Altough I have met a couple of Eleanors and what I initially heard was "Elena" (pronounced like Helena without the "h"), so I assume that's why many people think they are the same name.
I have just looked it up on the Oxford Names Companion, and it says: "Eleanor (variants Elinor, Ellenor) - English: from and Old French spelling of the Old Provençal name Alienor, which has been taken to be a derivative of Helen, but is probably of Germanic derivation (with a first element 'ali' - other, foreign). The names was introduced to England by Eleanor of Aquitaine".
Elena is simply defined as "Italian and Spanish verson of Helen". This one is Greek in origin and is pronounced 'ehn-LEH-na'
Ellena does not appear at all and it is a comprehensive book, so OP, to answer your question no, it's not a 'standard' spelling. To save hassle to your DD I'd go for 'Elena'.
Now Helena, Ellen, Elena, Helen are all the same name.
Haha, you guys were really confusing me for a moment until I realised that in most English accents, "Ell-en-a" and Eleanor are pronounced the same. In Ireland, Scotland and the English places (Gloucestershire is one I'm thinking of) where we pronounce the 'r' at the end of words, they're two totally different names!
But yeah OP, I don't think it's good to go with a non-standard spelling. If you like that pronunciation, go with Elena and just tell people how you're pronouncing it. 'Ell-en-a' is common enough that people won't freak out!
Ellena was on our shortlist for dd. We considered it because to me Eleanor is pronounced as it is spelt, eh-lin-or. Surely the pronunciation of ellena is obvious? Also considered Elena which is pronounced eh-lay-na, possibly spelt Alaina. Ellen is also lovely. If we have another dd both ellena and Ellen will be on the list again so I say go for it! :-)