The statute you quoted comes under the procedural section of Title 28. I admit, I didn't know it existed. I was only looking at the section which actually granted the authority to magistrates to hold individuals in contempt, which is silent as to any limitations. Neverthless, I don't see how this statute specifies anything different than what I have been saying all along.

Here is a quote taken from the US Supreme Court Case you just cited.

The paradigmatic coercive, civil contempt sanction, involves confining a contemnor indefinitely until he complies with an affirmative command... In these circumstances, the contemnor is able to purge the contempt and obtain his release by committing an affirmative act, and thus carries the keys of his prison in his own pocket.
This is the general standard for civil contempt and there is no limit on the time which you can be forced to serve.

But, as I said "civil contempt is purely coercive, the sentence can continue so long as it serves this purpose" which is exactly what section 1826 is referring to. If you are a witness than it no longer serves any purpose to confine you after the trial. But for anyone else it may. You can be held indefinitely even well after the trial. The guy who I had held in contempt that I mentioned, he wasn't even placed in contempt until after the trial was over.

But... I was wrong. I guess there is more express federal guidance in this area than I thought, at least regarding witnesses held in contempt as opposed to non-witnesses.