Again, you are assuming a usage that may never be the case.
What if someone wants one as a coffee table?
They haven’t agreed to detonation, they agreed to prop their feet on it and use it as a conversation piece.
Would this be a sin?
The problem with a reductio ad absurdum postion in an argument is that it doesn’t reflect reality.
What if I bought Playboy magazines in order to prop up a window that keeps dropping down on my house cat when she wants to look out the window? Or, using your example, what if I bought them in order to have ready table legs for a coffee table. What if I purchased illegal drugs as it was a quicker way to get plastic bags that I needed for some other use.
In the entire history of nuclear weapons no nation has bought them to make coffee tables out of them. They serve but one purpose.
The better argument is that modern nuclear weapons can be used in a way that isn’t disproportionate, and therefore isn’t automatically morally illicit. The reason that argument tends not to be made is that it suggest that our 1945 use was morally illicit, and that our deterrent use from the late Cold war through the early 1970s may have been morally illicit. That’s an uncomfortable topic for many, many people (I frankly think the 1945 use was morally illicit). We don’t really want to take on the current “they aren’t what they use to be argument” as that would mean that we might have to admit that what we were doing was wrong.
Indeed that would be real change, if there is one, to the Catechism. It already says their use was wrong. But if use is included to mean use as a deterrent, which we’ve never held before, that’s uncomfortable. But we have to admit, in the real word, use as a deterrent means you would use them.
Now, however, that might not mean what it once did, which is probably where the debate should be.