1. Can the machine be ground taxied to the rear?
Yes, quite easily.
2. What is the flight qualified aft hover taxi speed limit? OGE in particular.
30 Knots, combination of KGS and wind speed.
And, if you know, an associated question: what was the determining factor in the establishment of aft nacelle angle design limit?
Not sure what you're referring to with the question but the nacelles go aft to 97.5 degrees. I'm assuming the length of the jackscrew in the conversion actuator might have had some impact. Are you inferring that 97 degrees is not enough?