1. I think that signature bans is right punishment. I don't think we should tolerate copy-pasters up to certain level by giving them warning points.
2. Plagiarism is against forum rules and trust has nothing here.
3. Without strict enforcement this rule is useless more or less, but I think that we should leave it. Permanent bans means permanent, forever, I don't think that people should be allowed to start with new account.
I have different idea - statute of limitations (sorry if I used term incorrectly). If plagiarism was done very long time ago and user didn't repeated this mistake later, I think that he shouldn't be banned. It's same like if you make small crime long time ago, you can't be punished for it after 5 or 10 years.
4. Maybe we should leave this for moderators. Community voice may be used only as recommendation. For example if user who got banned was very useful for community and contributed a lot and many people are asking to unban him, maybe mods could consider.