The past years of witnessing an... "active" Supreme Court have gotten me pondering a lot about constitutions, powers and interpretation.

First question is: can a constitutional court be wrong about the Constitution? Does it have an absolute final say on the meaning of its words?
The obvious common sense answers are no and yes. Only the court's opinion has authority. A layman's opinion is just his opinion, and even a learned scholar's opinion is just an opinion. Even the lowliest judge has more authority than the highest scholar, and the only limit on his interpretation is an overruling by a higher authority (an appelate court). The Supreme Court has no higher authority.
This is obviously necessary because laymen and especially civil servants need legal stability and predictability. There are thousands of civil servants and they must all apply the law equally. If each of them has his own take on what is legal and what is not, the State collapses into anarchy. Hence they must all follow a single entity's interpretation. And there's also a citizen's gneeral duty to obey authority and channel his disagreement into legal methods to question and overturn authority.

But what if the Court says something outrageous? What if it rules the Earth is flat according to the Constitution? What if it revises the law on impeachment of Supreme Court justices to make them harder to impeach? (This one actually happened.)
Cheapest answer is that they wouldn't do it because the text's inherent meaning constrains their rulings. But this is just a speedbump. Philosophers demonstrate there's no such thing as an "un-interpreted" meaning of a text. With enough sophistry, any text can be made to mean anything. And the legal profession selects for the most devious sophists.

One can reply that only a limited set of interpretations can be reasonable, interpretation is never completely free. This is true. But who gets to say what is and is not reasonable? Only the Court has authority. If everyone finds their rulings unreasonable, that's just everyone's opinion.
A practical answer is that the Legislative and Executive will use their legal methods to restrain, impeach or change the composition of the Court. All of this still presumes that their flat-eartherist interpretation of the Constitution is right until overturned; their authority is still sacred.
And if it is sacred, what are you supposed to do if all such reactive measures by Congress or the President are ruled unconstitucional? What if the President names a new justice and the sitting justices nullify the appointment because the Constitution says justices must be qualified and they find him unqualified? What if Congress impeaches a justice and the Court strikes down the impeachment because the Constitution dictates a neverending march to utopia and Congress was merely disagreeing with the justice's utopian ideals?

Now those are silly made up examples, but if the Court's word is unquestionable, there's nothing you can do. The Constitution is whatever the Court says it is. And since everything from the tiniest private contract or local ordinance derives its legitimacy from the constitution, the Court has the unlimited power to bestow and withdraw legitimacy as it sees fit.
This is obviously false if you understand separation of powers and checks and balances. The courts ought to have their power checked by the Executive, Legislative and the original Constituent Power, which is above all three.
But what does it mean to check a power's decision? It is overriding it because it's illegal i.e. it is found to be incompatible when compared to the law. Those are two objects for comparison. The latter (the law) is not an independent standard but it is whatever the courts say it is. If the courts determine the two objects of comparison, they determine the result and there is no way to check them. What does it even mean to say the courts derive their power from the Constitution when they define what the Constitution is?