Great answer, now go ahead and expand.
Because you asked...
Why did MNU need his arm if they could have gotten the arm from any alien and forced it to fire the weapon just like they forced the dim-witted idiot to do?
Apparently, there was nothing special about the MNU guy either. They were going to cut the arm off before he escaped!
Christopher (the alien) said it would take longer than expected to reverse the process and turn him human again (a few years). MNU guy responds by knocking-out the alien and leaving him for the MNU guys to capture...destroying any chance he'd have of being normal again. He went to the hidden pod by himself, not knowing if he could pilot the craft. Where would he fly to, anyway?
After the pod is shot down, he runs away. Why, then, does he inexplicably fight his way back to it later on?
Gang members are holding MNU guy down on the ground so their leader can eat his alien arm. The gang leader has eaten many aliens, hoping it will give him the ability to fire the alien weapons. This has never worked and there's no indication that it ever will. MNU guy is fortunate enough to grab an alien weapon (which only he can fire). He blows away everyone around him...except the gang leader. The leader glares at MNU guy, vowing to kill him and eat the arm. Basic dialog: "I am unrelenting. I *will* kill you and eat your alien arm. You are in a position to solve the problem right now. If you don't kill me, I will kill you." Pop quiz! 1) What did MNU guy do? 2) What did Nigerian gang leader do?
After demonstrating how the mech armor is nearly invincible, why did he act
conflicted about saving Christopher (the alien)? Isn't Christopher the only hope MNU guy has of being normal again? Keep in mind, Christopher wouldn't have been captured if MNU guy hadn't done something unexplainable.
Fucking worse-than-Transformers awful.
[ADDED]
Why does alien "fuel" make a human transform into an alien?
If he was sprayed in the face, why did the transformation start with his hand/arm?
The aliens were trapped in their own mothership for weeks before humans cut through the hull and released them. They were malnourished. There was no food. The ship was completely disabled. When the ship first appeared, a smaller pod craft fell out of the ship and got buried in the ground. The alien and his son scrounged for years to collect enough of their own alien "fuel" for the pod to return to the mothership. Once there, the plan was to: 1) Travel to the alien homeworld, 2) cure MNU guy, and 3) come back to rescue the other stranded aliens.
...how did they plan to leave earth in a hopelessly disabled ship that no longer has food, weapons, or provisions?