On deadline you do not need a perfect forensic examination; you need a repeatable procedure. The order below takes minutes and leaves a record of what was established and what was not.
Ask for the original file, not a screenshot of a messenger window. One capture removes most of the layers a check depends on. If the original cannot be obtained, that fact is itself worth recording.
Do not read them as one lump. A signed capture with one region edited and an unsigned ordinary photograph are entirely different situations. Each axis has to be read for what it says.
Inconclusive is not a failure, it is information. Because what was missing is written down with it, you can decide whether to chase the original or verify another way. It does not mean the photograph cannot run — it means you know what you are running.
Export the report into the story's record and you can explain later what you were looking at when you decided. The hash travels with it, so you can also prove it was the same file.
This is general guidance, not legal advice. A scan result is a probabilistic assessment, not proof.