Regardless of whether you ship Ro/Ro or flat rack, it will be somebody at the port who lashes the Unimog. Some do a good job, some don't. While notes and the like might help, assume the batteries will be dead and also assume it will be impossible to jump start 24v at port. Of course you might get lucky but you should always have a plan for a tow out and bring a jump starter with your voltage.
Finally, Unirover points out the moot point.
How to lash down is only relevant to you if you are going to pick up yourself, with the hauling vehicle. My research into Wallenius-Wilhelmsen was that they lash to SPRUNG WEIGHT, rendering your own supplied ideas completely irrelevant. This method is because packing is so close, they can not allow vehicles to sway with the ship's motion.
For my money (and I did put it where my reply is), I would ship RoRo, no question. I would also take care to verify drivability, with the exporter, (fluids, portal seals etc), and then have trip permits in hand and go pick up the truck (with friend, chase vehicle, tools and supplies), and drive it home. Which is what I did do. Involves the fewest extraneous organizations and associated unthinking boobuses. KISS
Finally, I wonder if some folks on the site get kind of burnt out on the same questions. FWIW, here is the link to a good thread on the subject of ocean transport/ delivery:
Final thoughts: Find and use a GOOD customs broker, one who has brought in older vehicles before. And DO YOUR HOMEWORK on the POE, they all have differing rules and procedures; you do not want surprises post-landing.