I read this and, honestly, how could you prove that someone is actually capable of decrypting the disk?

I don't believe the Judge has to prove it to hold you in contempt of court and throw you in jail, the Judge only has to reasonably believe it. However, they can only hold you for so long before it gets reviewed again, but that's awhile - 2 years. And it can be renewed.

I think that's how it works.

Police in search of evidence, armed with a warrant, can break whatever they need to break.

Since their use is not infringing, and breaking the encryption is necessary to their legitimate use, DRM laws would not stop them.

No, the police are allowed to break the law to a certain extent in the pursuit of their duties (this is why they can speed and break down your door, the latter provided they have a warrant or probably cause).
Actually civil contempt varies by state, in some states another judge cannot overturn the civil contempt incarceration (Alabama, for example) and in some states there is no mandatory review period. Civil contempt has drawn fire in several states lately and since the judiciary wants to keep it as a valuable tool for enforcing court orders, there's actually some push to review it and put some requirements (e.g. mandatory review, max incarceration period, etc.) on it.