This is called the 'transept' and its purpose is to provide separation between the Sanctuary and the Nave. Sometimes the transept will include an area for the choir (although a loft is more common) and/or statuary, or side altars.
The Nave is the middle part of the Church. This is where the congregation sits during the service, and is flanked by aisles on each side.
If there is only one room, then it is just called "the chapel", if it is divided, the sanctuary is where the altar is, and the nave would be outside the sanctuary where there would be pews, etc. If there is an anteroom, it is called a narthex.
Nave
It isn't called anything. It is just called part of the nave.
It is called the cruciform. It is the combined shapes of the nave and transepts.
A nave
Nave is not an adjective. It is a noun for the middle part of a church.
Usually the walkway in Anglican Church would be known as the nave
If you mean the side arms of a church, it is called "the transept". Each of the two may contain a chapel.
No. A nave is the central area of a church, where the congregation sits.
the central open space within the church
the central open space within the church