That's why I said that US constitution is shit. Modern constitutions solve this in a different way, actually separating the judicial branch. It's normal that the first constitution didn't get it right. What's insane is that US system was never reformed. In other countries people celebrate constitutions that are just historical documents. In Poland for example "Constitution day" celebrates constitution from 1791 that was used only for couple of years. It would never occur to anyone to actually try to use this constitution today. Yet that's exactly what US is doing. They can't comprehend that some documents can be placed in a museum and still be respected.

The answer is relatively simple, the supreme court just kinda gave itself a shit tonne of power early on and nobody stopped them. Also the states were meant to work as a check on federal power as well, if this shit happened pre-1910s there's a solid chance the president would've been shot or otherwise DC would be under siege from Maryland and/or Virginia.

...can a president appoint themselves as a Supreme Justice, eliminate all the other justices, and then just be the head of the judiciary for life?

It should be a stupid idea, but recent events have been of little comfort.

No, constitutional tribunals decide if new laws are constitutional or not. They can throw away a law even before it's enacted or applied anywhere. They very much do prevent things.