The doctor was Crocus. Crocus was not at Twin Capes for 50 years. He was lying to Luffy. Notice that the captain he refers to in the flashback has his eyes shrouded. Second, notice that when Luffy leaves, and Crocus is speaking in general to Roger's spirit, he uses terms of familiarity. If he had been a nakama or a good friend or Roger, then he would not address Roger in such a manner.
Second, I strongly believe that Nico Robin's father is Gol D. Roger. Here's the evidence:
1. Roger's eyes are never shown. Now, Robin and Olivia are the only people in the Odaverse drawn with Irises. Robin's eyes are brown (No, the anime is wrong. If you look in color artwork they're brown, not blue) and Olivia's are blue. Now, far be it from me to assume things, but I think Roger's eyes are brown.
2. Roger could read the poneglyphs. If I recall, the only other people that could do that were from Ohara. Now, Roger is not from Ohara, but he more than likely had to learn the writing from someone FROM Ohara if they were the only people that could read it, eh?
3. The timeframe fits. When Olivia left Ohara, she was "Carrying on her late husband's research". Late husband. Notice that. Who had just been recently executed? Why, none other than Gol D. Roger.
4. Robin's devil fruit power. Now, far be it from me to say, but devil fruits aren't exactly things you find in the local market. That said, let's consider the possibilites:
A. Devil fruit came from a marine source. Highly unlikely as Robin became the target of the marines during the Ohara Buster Call.
B. Devil fruit randomly washed up on shore. Possible, but unlikely that a little girl would find it before someone else did.
C. Someone with fame and power (Let's say, a Pirate King), who is a smart man but not a very logical thinker, decides to give his daughter a Devil Fruit.
Now, Kokoro said Roger was stupid like Luffy. Now, Luffy is pretty dumb when it comes to thinking logically. Things are "Cool" or "Mysteries" to him. Yet, in combat and physics knowledge, he's pretty high up there. And it seems Roger has one up on Luffy by knowing the poneglyphs.
Final analysis? Roger gave his little girl a devil fruit and then left to discover the final information on the Lost Kingdom Raftel. Garp came cruising along and cornered him and his crew, for whatever reason, Roger gave up, and you know the rest of the story. Meanwhile, wife Olivia and little daughter Robin are safe in Ohara… for now.
