MC basically can do whatever he wants as long as he does not piss off another Noble family of higher status.
Nobility is law... heck even now, while not exactly "nobility", money allows most "crimes" to simply be a single cash settlement away from being expunged.
Especially if you commit the crime through an organization like a corporation - where the only actual punishment is fines... and fines only make something illegal if your poor.
The only other possible authority is the Church/religion, which in some more advanced governments where the "source" of "noble power" and superseded all - even royals... but the "laws" they needed to follow where pretty lax and often changed specifically to suit the needs of the nobility and only existed in Memetic space as a "check" on their power.
The Notion that Nobility had to "Answer to god" was a good way to keep the commoners "content"... even if in reality, any attempt to bring Nobility to account would be fruitless in almost all cases.