So if the device exploded somehow at the scene - how were the brothers not injured from this explosion - I assume they were holding the device to trigger it?
They were holding the remote control transmitters (e.g., in their pockets). So, they each set down their backpacks, then moseyed a safe distance away, and finally, coordinating via cell phone, pressed their respective remotes about 12 seconds apart.
What the FBI found was fragments of the remote control receivers, which the brothers had removed from model cars and wired to activate detonators (e.g., a broken flashlight bulb with filament intact or, the AQ Chef's favorite, a broken Christmas tree lamp). As for the FBI's analysis, I expect it was pretty much complete when the toy car company contacted them to say they recognized pieces of their product in the published photos. Freepers had also recognized the pieces as model car parts.
With their line of sight nonsense, the conspiracy nuts are trying to make this seem to need more moving parts than are actually present. Conspiracy buffs like moving parts.