In the early stages of the incident (before the RH engine was feathered) the desecnt rate was up to 1600 fpm without any corresponding speed increase. This suggests that the desecnt rate was caused by the drag of the mis behaving RH engine / yawing aircraft rather than poor airspeed / attitude control by the pilot.
Gear up, flaps up, one engine on cruise power, the other engine surging, descent rate of 1600 fpm without any corresponding speed increase - due to drag/yawing caused by surging engine??
Sorry, but I find that hard to believe.
Dr 
PS: Yes, I am endorsed on the PA31.