Usually it shows when the film was processed or printed. The picture could have been taken weeks or months earlier, or the print could be a later reprint.
No. They suggest an approximate range, because people keep clothes and vehicles for years. Combine several clues and express the result as a range.
It is valuable testimony but may have been written long afterward. Record it exactly, ask who wrote it and compare it with the physical evidence.
Treat it as one clue. Device clocks can be wrong, and metadata can be edited or removed when images are shared or uploaded.
Yes. Stamps, codes and handwriting on the back are often the strongest dating evidence, and they are lost if only the front is scanned.