1) If the BBT were valid, what banged?
The formless, and timeless quantum foam 'banged'. See next answer.
(2) And who or what triggered the blast/expansion?
Quantum uncertainty, the same phenomenon we observe readily today that results in Q phenomena. Your electronics would not work without it. Electronic devices and circuits in your CD players, PCs, DVRs, iPods etc. RELY on probabilistic quantum phenomena like quantum tunneling. End of discussion.
(3) Why did overall order not lasting chaos emerge from the hypothetical Big Bang?
Good question. Because the universe hyperinflated, smoothing out the quantum discontinuities.
Next cosmological question?