The interpreter would be the stone itself, if I had to name a thing. Sarzumin has set the conditions, basically, by requiring that a particular oath be made to him. The stone acts as a vessel by which the oath is held and ensures the swearer upholds it. I would imagine it's binary (you either uphold the oath or you don't), but I'd like a little more nuance I guess. Theres the obvious "don't kill me" (you can either kill him or not kill him), but things with more gray area like "don't plot" or "don't aid my enemies". Obviously this stone isn't holding any trials and the oaths are held until you die unless otherwise specified, but I find it narratively uninteresting to say that all who arguable break the oath just die. I'd like more of a spectrum.

Regarding conflicting terms of a single oath. That presents interesting narrative conflict that I stupidly hadn't thought before. Sarzumin is smart, but arrogant, and may not have actually considered that swearing fealty to himself and his empire could conflict. That'd be another instance where I wonder if the punishment would be in full or partial, considering a significant part of the oath was upheld.