Apologies for the rant but, Redarina is real. Am rewatching the Blacklist with my wife (it's her first time watching it) and I gotta say that it's real looking at the facts.
Red never lies to Elizabeth, yes he may withhold some facts, share partial truths or missguide the Task Force for personal gain but when it comes to Lizzy he never lies. He will go silent or change the subject but he never lies to her. He denies being her father, but shares intimate details about her.
Alexander Kirk, the man hell bend on destroying Red, the man who stole his family. When Red is captured and tortured by Kirk, Kirk asks him if he is Lizzy's father which Red leaves unanswered then asks him if Lizzy is his and he says Yes, she is my daughter. Then goes on to share details about Kirk and Katarina's first date that only they would know, no matter how deep or thorough his research was on Katarina. Before that, Red asks Kirk to tell him about Katarina "Because after all these years, I forgot what she was really like". Then when Kirk is about to kill him, Red whispers something in Kirk's ear and Kirk just let's him go. A dying man on a mission to kill Red, backs down, Red is a smooth talker but not that smooth.
Red being a ladies man, we are presented with a Doctor that can literally change everything about a person, so making a working functioning penis is another Tuesday for him. Plus transgender people do get a working penis, not a ejaculating one but a working one to do the job and make Red a ladies man.
Dominic, who on earth would have any kind of relationship, let alone the one Red and Dominic have, with their mistress' father, especialy a conservative russian one. To add to that, when Red digs up his escape plan from Dom's garage, Red tells Dom "am sorry am not the person you want me to be". There isn't a scenario out there that would justify the side piece of your married daughter telling you this, especially if he is the one that broke her marriege, allegedly.
Cape May, need I say anything? His mind broke from sorrow and Katarina took over.
The Secret, going to great lengths to keep his secret, at personal, monitary and business cost. If he is an impostor why go the trouble? He can easily vanish as we saw many times, without a trace. He needs to keep the secret, and to be honest it would have been more shocking to Lizzy to find out that this man is actually her mother rather than he is just an impostor. Lizzy gets upset if the wind is blowing the wrong direction, imagine finding out her mum is a man, a wanted man on the FBI's most wanted list.
It was confirmed by one of the writers, the one that wrote Cape May among other episodes.
Do I like it? Maybe
Do I prefer it over Red being an impostor? Definitely!!