Continuity mistake: When the waiter picks up the tray with the poisoned drink on it, it also has a bowl of fruit. When he gets to Rayna's table the bowl of fruit is gone. (00:55:20)
Continuity mistake: When Susan is fighting the assassin in the restaurant kitchen the assassin is wearing regular flats, but when she kicks the pile of dishes the flats have a strap around the ankle.
Continuity mistake: While Susan is on the moped chasing after the car, some moments we can see that her stilettos become flats, particularly when she rides over the ramp and shouts, "I am so bad-ass," and also after she shoots the gun out of the car passenger's hand.
Continuity mistake: In Rome, when Aldo leaves Susan at the hotel, we can see her suitcase in the back seat behind Aldo as he says, "I like to play the odds," and he hands her the card with his number, but then the suitcase just vanishes from the back seat, though she never had the chance to remove it, and when Aldo drives away Susan has the suitcase.
Continuity mistake: When Susan is fighting with De Luca at his villa she's wearing boots with heels, but when De Luca is running to his helicopter and Susan is chasing him she's wearing flat boots, then the heels reappear.
Continuity mistake: When Susan meets Rayna face to face and prevents her from drinking the poison, the glass moves on the table from oddly far away from Rayna to the middle of the table between shots though nobody has touched it.
Continuity mistake: When the man has a hole burned through his throat, in one shot he has one hand on his stomach and the other by his side, but in the very next shot both hands are on his throat.
Answer: It's never fully explained in the movie, but it seems more likely that Rayna simply had a lot of enemies and people we're trying to kill her for various motives, but not to steal the nuke. For example, the guy who tried to poison her seems to do it out of spite for Rayna. The steward on the plane seems more intent on kidnapping Rayna to get what she's selling (although the pilot before dying says "Stan and I already sold it...", so he may not be after the bomb). Karen Walker, who was a double agent who sold Rayna the CIA names, is more likely trying to kill Susan to avoid being found out and not Rayna. Although if Karen was trying to kill Rayna, she may have already discovered the bomb's location, through various means.
Bishop73
Right, when in the basement, to make Susan believe him, Fine asks her "Did (or didn't) Karen try to kill you?" And Susan realises he had sniped Karen and her driver to save her from being killed by Karen.