It helps, but supplement it with a saved page or PDF, the post address and the exact timestamp, and keep the original files unchanged.
Generally not. That data is held by the platform; tell the investigating agency if important content may exist only there.
Open the individual post or hover over the relative time display, and record the exact timestamp along with your own time zone.
Save only what your research needs, store it securely and avoid republishing speculation or personal details about uninvolved commenters.
Give the platform, account name, post date, address and capture date, and note whether the post is still live or has changed.