1) The cops have a warrant to search my computer. They’re demanding my encryption key. Do I have to give it to them?
No, there are no circumstances under which you can be forced to give up an encryption key because it violates your constitutional right to due process.
No, you don’t have to give the encryption key to the police as they seize the computer, but it’s possible a court could make you do it later.
Yes, if the police have a warrant for your computer, it includes your encryption key. If you don’t tell them, you could be charged with obstructing justice.
Yes, in fact you should always give your encryption key to the police, even if they don’t have a warrant. It could help them in an investigation.