The "U.S. Presidential Succession" list shown in the Situation Room's big board is erroneous. The list is accurate only for the first seven listings, up to the Secretary of Defense. The remainder of the list should continue with the rest of the President's cabinet but does not.
While it is possible to give a casualty estimate for a nuclear strike based on the target alone, to give a more meaningful estimate, you also would want to know the yield (explosive power) of the weapon and the altitude at which it is detonated at the very least. Without this data, the estimates can be off by a factor of several millions.
It would've been fairly easy to identify which country launched the ICBM. All missiles have unique flight signatures. Everything from the plume to the speed to the type of missile technology used is unique to each country. An ICBM launched from a sub is also completely different than one launched from land. They could have used a spy satellite to confirm the origin of the launch within the time frame given missiles leave long lasting vapor plumes that are easy to detect and trace.
The president is not typically visible while riding in the presidential limousine because the windows are heavily tinted and armored to provide security. Furthermore, the specific limousine carrying the president is often disguised with decoy vehicles in the motorcade, and its position is deliberately varied to make it difficult to target.
In the third act, while the President is being rushed away in his limo, he looks out the window to see a child in an adjacent vehicle smiling at him. The presidential motorcade runs along a secured route escorted by police and secret service, no civilian cars are allowed to approach or be near the President for security reasons.
In one of the scenes inside the limo, a view out the windshield shows the vehicle navigating a moderate curve at high speed. The driver has the steering wheel turned a full 180 degrees, revealing that the view out of the car is green screen, otherwise they would have driven off the road immediately.
It is very interesting that during the short time-line of this movie, it is daylight in Alaska, and it is simultaneously daylight in Washington DC and in Africa.
In the absence of any threat against the American nuclear deterrent, the nuclear command authority would very definitely put a counterstrike on hold for several hours at least, by default, because in the given scenario, they do not even know which nuclear power is attacking the USA. Once the nuclear event has occurred though, they can then have the fission products analyzed and compared to previous analyses from known sources, previous foreign nuclear tests, fission products in the exhaust of foreign nuclear facilities and such. All this data is on file. It would very probably at least reveal the source of the bomb fuel with great certainty.
When one of the two launched GBIs fails to deploy and the enemy weapon is still in range of interception, there is not an immediate decision to fire two more while this remains an option. Nor does anyone speak up to suggest this. Everyone just waits to see what the second GBI will do, understanding full well by the time it misses, if it misses, the enemy weapon will be outside of the engagement zone.
The President and the stratcom general are clearly wrong to insist on an immediate decision about a counterstrike. When The President says "If I do what you suggested, I let whoever did this get away with it." That does not make sense. No American forces are threatened at all, only a single American city. Whatever happens to the city cannot significantly degrade the readiness of the American nuclear deterrent and indeed the American military. It is not a choice of "use them or lose them". The decision to retaliate can therefore easily be postponed as long as the nuclear deterrent is not under direct threat to be wiped out. Hours, days, weeks or months obviously do not make any difference.
The use of Site R (Raven Rock) is overplayed as a relocation site. It is the location of the ANMCC (Alternate National Military Command Center), but there is no mention of the FEMA Special Facility on Mount Weather in Virginia, which was optimized for the non-military agencies.