Is it possible to request it never be published, unless a trade goes sideways and its needed/one of the two/two+ parties requests it?
No, that would allow for loopholes like this:
if a potential scam victim receives the PM as BCC, I can't know everyone involved.
Is it possible to make this program entirely encrypted until the delay is up/ publishing requested ? (Not sure if this would even be possible).
Only if you send an encrypted PM, and publish the decryption later. But that defeats the purpose of using a third party to publish the evidence.