But you’re exactly right. They are contradictory and that’s the point of it all.
Silent Assassin is to just kill your targets unseen – you do that, perfect, you get a whopping 30Merces.
You don’t do that, you get a 10Merces payout instead – if you can kill guards and escape.
You’re trying to have both cakes and eat them too, but that’s not why the objectives are built the way they are, IMO.
They push players to take more risks against NPCs that have guns, to take more time in the level – time they may not have, if in an area with lots of enforcers.
It’s a roguelike. It’s supposed to work against you. Not getting easy Silent Assassins, no easy ways to make LOADS of money by having SA-Prestige-friendly objectives like that.