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If you had an uncrackable safe could they compel you to open it though?

Yes. They can't compel you to give them the means to open it (e.g. tell them the combination), but you can be forced to open it for them.

Then if you are guilty say of murder, the best strategy is to refuse. In that case, you'd be only liable for contempt of court.

That depends on whether they can hold you indefinitely for contempt of court. It has been my understanding that in some jurisdictions, they can.