It looked like a real gun, so it was completely reasonable for the defender and the other people in the taqueria to be in fear for their lives.
Correct. The reasonable belief principle applies here. A reasonable person in the place of the victims would believe they were facing an armed robber.
How would the defender know whether the robber was unconscious vs playing possum?
He wouldn't initially, but there are two important factors to consider.
1. After the CCW walked up and shot the guy multiple times at close range and the guy didn't move at all, that's a really good indicator that the guy either isn't conscious, isn't alive, or can't move. Any of those three things mean the guy is not a threat.
2. After the CCW also picked up the guy's gun it's just about impossible to argue that the guy still posed a threat.
Once an attacker is down, not moving and there's no longer a good reason to assume they are armed, it's going to be really difficult to legally justify shooting them in the back of the head at point blank range.
The same principle of reasonable belief applies. If the defense attorney can convince the jury that a reasonable person would consider a person who has been shot multiple times, who is lying on the ground and not moving to still be a threat after they had also been disarmed, then the CCW walks. Might be possible, but it won't be cheap and the odds aren't something that a sane person would want to risk.
Keep in mind that we aren't trying to exonerate the CCW, we're trying to learn lessons. The big lesson here is that it's important to understand when to stop shooting.