> But what counts as "innocuous or obviously benign" was never established. And this "innocuous or obviously benign" line is EXACTLY what distinguished between whether a gratitude was accepted with a corrupt state of mind.

Easy: someone would complain and a court would decide based on the specifics of the situation. Most laws work this way and cannot actually resolve based on a programmatic list of facts.

> Not really a good example, because unless that's something like a theater performance there is basically no way forward from this, which could end with the teacher handing out good grades and receiving a mug from these students without this scenario becoming bribery.

Are you arguing that grading (outside of "something like a theater performance") is fully objective? Because... it's not.