Hi, Mark
I assume you have sorted your issues out, but let me add my 2 pennies to the questions for benefit of others potentially seeing this thread.
1. The plug is indeed the same as Yaesu in FT817 or VX handheld series – it is 4.0/1.7mm plug
2. Pico Paddle is best as it fits naturally the side of the rig, and moreover the base mounting hole matches exactly the side screw place in your MTR3b. You just need to replace the manufacturer’s screw with a slightly longer one as you add the base of Pico Palm paddle.