I think trust ratings should be visible on all boards for these reasons.
Firstly before I reply to your suggestions, I just want to inform you not to rely on the forum when it comes to tackling scam related activities. The forum doesn't moderate scam.
For A,B & D,
Safeguarding yourself from scam related activities should be your priority don't ever think it's that of the forum.
C) Plagiarism is a form of exploration of other people's work for personal gain that deserves to be qualified as a scam that can be exploited on any board.
It already has a punishment which involves banning the culprits, leaving red trust isn't necessary. Just report with valid evidence to moderators. Trust system should be used for issue relating to trades especially when it has to do with the negative feedbacks