>>18077990
Again, the simple answer is that we can't know for sure.
It is a Romanized Germanic who made that inscription, so for all we know he wasn't referring to any specific Germanic god, but rather to Roman Mars. Romanized Germanics were erecting idols to Jupiter and Mars for centuries in full Roman manner and were essentially fully integrated into Roman culture.
btw I am familiar with your posted inscription, but are there any other references to Mars Thingsus besides it? A sample size of one is not much to go off of.
>aren't associated with assemblies and law-making bodies
At least not in Iceland a thousand years later. Remember that the Edda is not collected from all across the Nordic world, but just Iceland, and quite late. Absence of evidence is not evidence of absence. Any number of Germanic gods could have been associated with the Thing by any number of different Germanic tribes.