Hypothetically speaking, could I bring a case to the magistrates on behalf of someone other than myself?
You can, and in fact are encouraged to, bring cases even if you are not affected. If you see someone violating the right of another player, do speak up on behalf of that other player. He may not dare to or know how.
What happens if defendants of the case are themselves Magistrates?
They would probably have to recuse themselves and act only as the defendant.
And third, do forum posts fall under the jurisdiction of the magistrates, and under the Social Contract?
I would say all activities done on official channels (forum, wiki, in game), are subject to the social contract. As Magistrates have a mandate to settle PvP disputes, it would likely fall in their jurisdiction. However, I don't see any precedent, and the mechanism for complaints exists only ingame. I'd wait to see what Tom has to say about it.
