I think that Nikolaos and Tatiana will marry , I think this might be soon
Coming from a very knowledgable position, I would be surprised if this happens at all. The simple reason I think is that Tatiana's family has too many skeletons in the closet on which the media would have a feeding frenzy. This would not be good for the Greek Monarchy.
The common denominator here and the key to the whole mystery is Venezuela. I am currently writing a book on Tatiana's extended family (with a particular focus on Tatiana's maternal grandmother the former Countess Elinka von Einsiedel who married several times with different children by different husbands, and now is a widow living in Marbella, Spain. - about the Venezuelan, American and Spanish connections. Tatiana's Father, of Hungarian origin, now deceased, made his fortune there, her stepfather, now living between New York and Switzerland also made his fortune in Venezuela. Tatiana's mother, Blanca, spent time in Venezuela and the book will elaborate on this and her Mother as well, and more recently her Uncle was killed there in very mysterious circumstances a year ago. Her sole surviving uncle still lives there.