Well, until the IA investigation is completed, I wouldn't jump to any conclusions.
1. There are a lot of people who can Cirque du Soleil their body so they can move their wrists under their butt to get the cuffs in front of the body.
2. He could have used his right hand to get a hidden gun from his right ankle, and while fumbling with it, he shot himself in the right temple. Just because one is left-handed doesn't mean you can't shoot with your right hand.
3. Guns comes in different sizes and he could have had a small derringer, not a 44 magnum desert eagle. So, a derringer could have been missed in a sloppy search, i.e. they searched pockets but not the waistband. For the real thugs, inner waistband is a popular place to put a gun or contraband, not the pockets.
4. He may have had a derringer in his ankle, and he was trying to get rid of it since the cops didn't find in. Since his hands were tied up, he used his other foot to get the gun out. Since a derringer doesn't have a trigger guard, his toe pushed the trigger and his head was sideways as he was trying to work the gun out and it went off.