>So "I know stuff about a thing that you have a lawful order to investigate, and you don't, so any crime of which I'm suspected is now uninvestigatable, nanny nanny boo boo" is fine, real and serious
No, this isn't fine. If there is a lawful order to gather evidence related to the crime, the accused can be compelled to comply. For example, the accused can be compelled to provide documentation under subpoena, submit to police interrogations, be a witness in court, etc.
They cannot be compelled to testify against themselves. When someone invokes the Fifth Amendment, the presiding officer must decide whether there is a reasonable claim that responding to the question or demand as posed may incriminate that person. If the presiding officer is not satisfied that such a connection exists or that the inculpatory value of the response does not rise to the level of testimonial self-incrimination, they can, and do, order the person to comply. If the accused feels this order has been wrongly made and that their Fifth Amendment rights either were or could be violated, they are free to challenge the order in court, but that doesn't mean that there won't be consequences for non-compliance while the challenge processes (a judge overseeing the challenge may or may not issue a stay or restraining order to limit such consequences).
>Should you be also compelled by indefinite arrest to disclose location of the body of your victim (when other evidence against you is insufficient)?
No, because that unquestionably rises to the level of testimonial self-incrimination, whereas granting access to something found during the execution of a legal search warrant does not.
>Because it looks mighty similar to disclosing password to encrypted evidence of the crime.
Ultimately the courts will have to decide whether this is similar enough or not. Under the current law, the answer is non-obvious, and it must be established by the conventional legal process, which yes, is indeed pain-stakingly slow. But there is no exceptional injustice underway here. This is the way the system works. It's not always pleasant, but then, neither is the real world.
If you want to discuss reforms that could require all cases to reach a final resolution within 12 months of their initiation, I'm all for that. Dragging these proceedings out over 5-10 years is very frustrating. But again, the backlog and slow conventions of the courts are a different matter than whether or not this man's constitutional rights are being violated (except insofar as these apply to the 6th Amendment, which guarantees the right to a speedy trial; it does not, however, guarantee the right to a speedy appeal), whether his detainment qualifies as "torture", or whether some other grave miscarriage of justice is occurring here.