>Having read some additional sources, the justification for forcing the three individuals to speak is that they were given immunity from prosecution by the grand jury. This supposedly prevents them from taking the fifth, because they can't incriminate themselves. That smells like an attempted end run around the constitution.

This is pretty standard and has been for (at least) decades. The constitutional injunction is against self incrimination. You can be forced to testify against others, and a whole lot of people have been thrown in jail over the years for refusing to do so.

>Freedom of speech must include the freedom to remain silent.

Why would you think that?

>Fourth amendment protections against search and seizure should apply as well.

This is beyond weak as a constitutional argument.