Should posts hold any legal weight?

Yes

yes

Yes, if relevant to the offense.. It could conceivably be used by the defense as well. .

if it's relevant and in a specific time period before or even after showing some signs of intention or confession then yes

but realistically, a criminal would want to keep their identity hidden, and social media is a dead giveaway

It should be used the same as a recording of someone saying something would

Nah

no

Depends