You can replay the missions
True but replaying the missions just creates a "restore point" and you go back to the mission. The thing is the assassinations have a direct result on the company of the person you kill, and their competition. Eg killing the 4 cigarette jurors makes the stock if the competing company rise 300%. But you have to make those stock plays prior (or immediately after) the hit. Once the mission replay is over, the game "respawns" to where ever you were and the stock market will be in current "real" time rather than directly post-killing.
Anything involving Lester needs to be listened to for potential stock market investment strategy if you want to get onto that level of realism.