Okay,now that we've established (1) that these Civil War amendments contain new delegations of power to Congress (e.g., Section 5 of the Fourteenth Amendment) and (2) that Congress is only empowered by these amendments to fashion legislation that is "appropriate" to enforce the amendments, we come to another fun issue.
What constitutional mechanisms, if any, exist to ensure that Congress does not enact legislation in excess of the powers that have been delegated to it by these amendments?
The same mechanisms that exist to ensure they don't enact legislation excess of any of its constitutional powers. Someone raises a challenge by defying their law, and the court (usually an appeals court, I would imagine) makes a ruling whether or not the law in question is indeed constitutional. Yes, that means judges have to get involved at some point. That's not what I have a problem with. I can even forgive them if their legal reasoning is different from mine. But certain things I consider unacceptable are: deciding a case based on whether or not they think it would be good for society; "compelling government interest"; saying that the Constitution is a "living" document, and therefore has to "adapt" to our changing "needs"; making rulings based on whether or not the results thereof would fit in with what was supposedly envisioned by the draftsmen (such as racial voting statistics); failing to delineate their legal reasoning, instead resorting to airy-fairy "principles"; to name a few.