Who is the Mastermind? The Bank Manager
The clues that revealed that it was the Bank Manager:
“He faces forward. The large front window facing the street is to his left.”
“The glass slivers from the exploding front window weren’t as kind. The right side of his face resembles a porcupine;”
What was his plan?
The Bank Manager had inserted glass slivers into his face earlier that day. As a victim of the caper, he was sure that no one would expect him.
He never had a cold. That was a ploy so as not to attract suspicion when he entered the bank with the jacket hood covering his face.
He was too smart to expose himself to the SUV crashing into his office. He hid under his desk.
Why his plan failed?
Given the direction he was facing, the glass slivers would have entered the left side of his face. As smart as he was, his reflection in the mirror got the best of him.