It's really a process of letting the subpoenaed know that they either tell the truth, lie and face perjury charges, or refuse and face contempt or court charges. The latter can seemingly land you in jail in perpetuity. Because fuck you, I guess?
That's mostly for police. Once you're in court and ordered to testify, the person talking about germany is mostly correct. You can't be forced to self-incriminate nor testify against a spouse. Otherwise yes. Generally 99% of courts won't bother even asking the defendant to testify because self-incrimination is practically guaranteed. Usually only if the defense calls on them, which is often a bad idea.
Only if there’s a risk at incriminating yourself, and if it’s not immediately apparent how you’d run that risk (e.g. you’re a witness that doesn’t have a direct relation to the crime at hand) you’d have to motivate how it could be incriminating.
You mean by a court subpoena? If so then you testify or get found in contempt of court.
Or do you mean what if someone is threatened/blackmailed into giving false testimony? If that's the case then you should probably go to the police. If it's law enforcement who are coercing you then I suppose you could try to include that fact in the testimony, but there may not be much difference in that and refusing to comply with the blackmailer in the first place, in terms of your safety.
If you're coerced to lie under oath then I'd guess that still counts as perjury, but I doubt most judges would be mad at you for it; they'd shit fury all over whoever was coercing you.
IDK, imprisoning a person until they either comply or the trial concludes without them seems pretty good for the judge. Bad for the person subpoenaed, but it's no skin of the judge's back