I find "Last Enemy" very useful, especially in city maps where the enemy has a tendency to get stuck in walls. It saves a lot of time having it turned on, you don't have to run around looking for that missing guy you missed in your first sweep. What the setting does is make the last man standing seek you out instead of you seeking him.
The Taxi diver is removed, he was not needed anymore since I changed so much in the startup of the game. The car has always been in that location, people just started noticing it when the taxi driver appeared
The mercs wont work for you if you have too low professionalship, and since you decided to kill Ali, you got a 50 point drop in that. That's the reason you couldn't hire any mercs after killing him, and you need to do some missions to raise the proffesionalship before anyone will work with you again.
EDIT: Ali only works with black people, if you're using mercs with any other nationality he wont join your team.