Due to the physical characteristic, all the NOR flash have life time with the limit program/erase cycles, which called endurance. Endurance is related with the program/erase cycles and temperature. With the higher program/erase cycles, the data retention (time of the cells can hold the data) will be worse than before, if your program/erase operations with very short interval and many times on one specific sector, it will accelerate the aging speed of this sector.
For your questions, please see the comments as follow:
1.- How do you expect this sector will behave (in terms of malfunctioning) when writing and reading?
--------The data read/written will be abnormal with incorrect value, and the time for the cell to retain the programmed data will be extremely shorter than the normal sectors
2.- Will this defective sector have any influence in the other sectors that have not been used?
--------The endurance and data retention is the "sector based" conception, the defect sector will not effect other/neighbor sectors from the physical perspective.
For more information, please refer our KBAs and AN from the following links:
Customer Application Engineer, Cypress