Well, D'OH!
There is absolutely NO downside to refusing to testify, for the express purpose of avoiding self incrimination.
If there was no possibility of self-incrimination, only physical evidence can determine if a crime has been committed.
I assume that testifiers are questioned under oath.
If the purpose of invoking the Fifth is to avoid incriminating others, then perjury has been committed.
Isn't that also a serious crime?
Setting up a private server is not a crime.
Directing someone else to set up a private server, knowing that it IS a crime, is prosecutable.
Setting up a private server knowing its purpose was to be a conduit for the selling of national security secrets might be a crime.