I've had the belief that Tiamat and Bahamut where the same for along time. If you listen to the way Bahamut talks in the Research Center he seems very bitter towards all of Humanity. Perhaps he began to abandon his old life in hope to seek revenge on humanity (for whatever reason). Thus changing his name from Bahamut to Tiamat leaving no trace of his former life. This new Dark Bahamut would also explain why his attack changes from Mega Flare to Dark Flare because he had embraced the Darkness within him. Also with this new Darkness within him it would be very easy for Ultimetia to take contol of Tiamat making some kind of false promise. It fits into everything in FFVIII that talks about Tiamat or Bahamut.

Oh and the reason that Selphie can't remember the name of her GF appears to more for the effect of how much GFs make you forget things not to implant some kind of secret, mysterious, GF into the game.