Okay. At the end of season 7, they knew they were going to ceate a spinoff of Atlantis. That is the real reason for the two part finale. Jessica Steen, who has been in many other shows, was tapped to play the part. When they actually got the casting of the new series, they decided to get another person to play the role and they choose Torri Higginson. Both Jessica Steen from season 7 and Hginson from season 8 and Atlantis are the same character.
As for the guy trying to leave earth. That was Col. Frank Simmons, played by John De Lancie. He was a part of the NID, the bad part of the NID actually. he also played Q in TNG, DS9 and Voyager.