Corrected entry: Callahan had obviously been planning his revenge for a long time, and the microbots were a huge part of the revenge. But if Hiro hadn't invented the microbots, then the entire movie wouldn't have existed. So Callahan basically just was lucky that a kid invented these amazing things, that were somehow just perfect to get revenge.
Corrected entry: When Hiro is showing off his Micro-Bots at the SFTT he puts the metal headband thingy on and the Micro-Bots start constructing themselves. Then when he takes it off all the Micro-Bots fall to the ground. However, near the end of the movie Callaghan is using the Micro-Bots to push Hiro and Baymax into the teleporter, without any help from the metal headband.
Correction: The control headband is mounted in the Kabuki mask Callaghan is wearing.
Corrected entry: When Honey first meets Hiro, she is rolling a solid ball of Tungsten Carbide that is about 2-3' in diameter. Honey claims the ball is 400 lbs. Tungsten Carbide is very dense, around the density of gold, but much harder. The specific density is 0.54 lbs per cubic inch. If the sphere was only 2 feet in diameter - it would weigh approximately 3,619 lbs - or about 1.5 tons. If it was 3 feet in diameter it would weigh an astonishing 12,200 lbs - or just over 6 tons. Honey would not be able to roll it around on the floor as easily as she appears to do in the movie.
Correction: The ball could be hollow.
Corrected entry: When Hiro and his friends are watching the footage of Krei's failed teleportation experiment, Callaghan is nowhere to be seen in it until they watch it later on at Hiro's garage.
Correction: Because Callaghan was way in the back off camera when they first watched it. After the accident Callaghan runs into camera view to assault Krei.
Correction: Callahan was plotting his revenge and got lucky when Hiro showed up with his microbots. He saw the advantage they gave and plotted to steal them, faking his death in the process. It's not a plot hole, it is just the plot. If the microbots weren't available he would have used some other technology.