Yes, I also think that a union between them would have been practically impossible. Neither or both of them were unavailable by the time they met. Also, Nicholas was eight years younger than Ella, making him the same age as her youngest sister, Alix. Had a union been possible, I think the fact that Ella was several years his senior would have been disapproved.
I know that Christopher Warwick touched upon the subject briefly where he included some excerpts of her letters to Nicholas and said that "Sergei's nature must have lacked flirtatiousness and romance, that of which she found, albeit fleetingly, with Nicholas of Greece, having an unexpected effect on and arousing feelings in Ella which took her by surprise." Hugo Mager also apparently touched upon the subject but I personally have never read his book. Joseph Howard Tyson in Fifty-Seven Years of Russian Madness also said she grew particularly fond of him as he amused her with his repartee, antics and humourous drawings and that there was an innocent attraction between the two, both he and Warwick stating that it was arguably the closest Ella would ever come to being unfaithful.
I also doubt that either one was in love with the other, but the excerpts she wrote to him were very tender. It may not have been an extramarital, passionate love affair but to me, personally, it is suggestive that, while she did truly love her husband, there may have been something severely lacking in her relationship with Sergei.
Anyway, I was wondering if anyone had the full transcripts of her correspondence with Nicholas? Or any pictures of the two together? Although I understand this may be unlikely given that they only spent a short amount of time together...