Instead of removing a signature, it can contain some public shaming saying it's removed because of plagiarism and user has to earn xx more Merit before it's enabled again.
Yes.
And only apply that to users who actually have a signature to lose, e.g. Sr. and up. Lower ranks should stay permabanned. Most plagiarism is done by newbies so they wouldn't be deterred by a sig ban.
I would imagine many shitposters would simply abandon such accounts and try to buy new ones.
a.) How often the user has plagiarized (and the last copied and pasted post)
There is no feasible way to detect that.
b.) The reputation of the user and/or time spent on the forum
c.) Values that user adds to forum (Why or why not they should be allowed)
Highly subjective and would just devolve into massive flame wars.