The MiG-15 neither has aerial refueling capability, nor would it be likely that a Soviet pilot would fly all the way to Las Vegas, almost three times the MiG-15's range (the closest Soviet ally being Cuba).
Holding a diplomatic passport does not automatically grant diplomatic immunity.
The NSA specializes in data interception and analysis (SIGINT, signals intelligence) and would neither be involved in debriefing a defector, nor inspecting a Soviet jet aircraft or making decisions on trading captured personnel with the Soviet Union.
The characteristics of the MiG-15, an already aging first generation jet fighter in the 1960s, were well known at that time. The first working examples were flown to South Korea (from North Korea) and Denmark (from Poland) back in 1953 and thoroughly inspected. Contrary to Carruthers' claim, a willingly defecting Soviet Air Force colonel and the intelligence he could provide, would therefore be far more valuable than yet another MiG-15.