Or, what if the time saved by not having to fight Alexius (who we quickly kill while he is distracted by the feigned return of his son) allowed us to complete the ritual before the demons could arrive to stop us, thus possibly saving the lives of Sera and Iron Bull?
What if Alexius could be convinced to help fend off the demons in exchange for his son's continued existence? Who says we actually have to release Felix if Alexius gives up the amulet?
Saying "either way, we get the amulet" is just thinking with video game logic, not pragmatism.
Pragmatism usually means thinking of the best way to get something done with the least amount of risk,for example there are a couple of robbers who are holding up a bank,you are armed and they aren't looking your way.
What do you do in this situation?
Option A:Wait until the police come despite knowing that the robbers will take hostages including you.
Option B:Get into a shoot out and risk being killed because they have the numbers advantage.
Or Option C:Wait until the robbers have their guard down and then quickly pull out your weapon and shoot them dead.
Either of those options have risks but only one has little to none.