“Mara” is a destination name, not a precise description of your daily movements. Two camps described as being in the Maasai Mara can give you different access, driving patterns and activity choices.
Adding Samburu to a Mara safari is a decision about contrast. You are moving between northern Kenya and the Mara's grassland setting, with different landscapes, wildlife interests and travel logistics.
The photograph is familiar: elephants moving across open ground with Kilimanjaro behind them. It is a compelling image of Amboseli, but it combines several things a traveller cannot arrange to order—animal position, light and a clear view of the mountain.
Nairobi National Park is close enough to the city to be considered around a travel day, but proximity does not make it a risk-free way to fill the hours before a flight.