Why is such a high US national debt not resulting in another great depression?
You answered question (1) with your own chart. The United States had a “high” debt-to-GDP ratio in World War II - and there was no subsequent depression. (There was a sharp recession as a result of demobilisation, but that was short-lived.)

For a country that borrows in its own currency, there is no obvious reason how a high debt-to-GDP creates recession risk.

There is no easy answer to (2). We are currently headed into the greatest economic contraction since the Great Depression, and the cause was a pandemic, which would not show up in a standard mathematical economic model.