Y'all are making it far more complicated than it is.
Due to comonality of parts (primarily the firewall), the conversion is fairly straight forward to drop in a 5 speed. The biggest problems are with the computer & wiring, not with the mechanical linkages.
Manual transmissions were not offered to the consumer market for whatever reason that Isuzu had at the time. The VX is similar enough though to Troopers & Amigos that the manual transmission swap is feasible.
I think they wanted to own" only full-time computer-controlled four wheel drive" for the VX. That was the marketing concept to enhance SUV handling.