A: How do you determine what wire is the RESET on a DKU-5?
You trace the wire back to the chip and check against the chip's datasheet (or Google for relevant sources). There apperars to be as many variations on this cable however as there are suppliers.
B: What are the ramifications of using the 3 core cable with just VCC/TX/RX?
I doubt you will ever find such a cable. The 3 wire GND/RX/TX however will enable Arduino to USB serial communication, but without auto-reset or power from USB.