I've often decribed the difference between ethics and morality in this way: ethics are applied, morality is imposed. That distinction is important, because ethics, in my view, are a set of guidelines that inform our behavior, where morality directs it. In that respect, all ethics are "situational." If you are in this set of circumstances, how do you behave?
I think your analogy to the Constitution is particularly apt, and why Scalia, and his "originalists", are inherently wrong. The founders were ethicists, not moralists. They believed that they had set up a system that could adapt over time to changing circumstances, and for hundreds of years that is how it has worked. (The "common law", with which they were intimately familiar, works the same way.) It is how we have progressed as a nation with the same Constitution.
The Constitution is a document intended to be applied, not a rigid moral tome. It is thus, pragmatic and ethical.
A well reasoned argument is like a diamond: impervious to corruption and crystal clear - and infinitely rarer.
Here, as elsewhere, people are outraged at what feels like a rigged game -- an economy that won't respond, a democracy that won't listen, and a financial sector that holds all the cards. - Robert Reich