most of the matters are now self evident,however after reading all of this its clear the PF's status as impaired from early on and 'twas unrecognised.
1.... distrustfull of the Artificial Horizon at a time when it was his only reliable indication apart from altitude.
2. Degrading into spatial disorientation, under psychological pressure, whilst unfortunately retaining control of the aircraft. The saddest part was the Pf's comment that "it feels fast", which ,if one was repeatedly pulling back on the controls to slow the aircraft, sounds very somatogravic....