When I lived in Japan (around 15 years ago, so etiquette might have changed since then) it was common to take the fish off of the rice and dip it in soy sauce, then put it back on the rice bed in instances where it was just placed atop the rice. Likewise, it was perfectly fine to mix wasabi into your soy sauce.
I've done things that way since without any overt disdain, so I think these are generally good guidelines, but you can probably get away with doing some things your own way.
Sounds about right. I have seen similar markers in California to measure fault line movement on an annual basis, as well as several in Yellowstone for roughly similar reasons. Seems like this is a survey marker to measure distance or alignment.