> What stops a President from simply choosing to ignore a Supreme Court ruling and what prevents the King from returning to personal rule?

Legally? The fact that everybody under the president -- including those in the military -- understand they are swearing their oath to the constitution -- not the King, not the Crown, not God, not the Supreme Court, not anything else. And that the Supreme Court says what the constitution means. And that if there is a clear and direct contradiction between the Supreme Court and the president, the former trumps (no pun intended) the latter.

Physically? "Nothing", yeah. Same goes for non-presidents. If you can get enough people to follow you (or maybe at least enough of the people with guns) everything else becomes irrelevant, including whether your title was president or King or God or Constitution or whatever.

> The lack of arbitrary rule is a defining feature of both

It is emphatically not. There are lots of countries with constitutions that nevertheless have arbitrary rule. As there are countries without constitutions or arbitrary rule.

> They are so similar as to be almost the same and if an 'indisputable source of truth' exists anywhere, it is not in the written documents or their structure but unwritten norms and rituals sit beneath both.

No, that's exactly what those are not. Unwritten stories, traditions, and rituals are very much disputable. That's kind of the entire point of writing things down, and the point of the game we call Telephone. The indisputable bits are physical artifacts everyone can see with their own eyes.