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The fifth only protects you from self-incrimination.

I think the fifth is interpreted more broadly than that. You don't need to have a specific law in mind that you believe you have broken (and would reveal) to take the fifth; you merely need to believe that it is possibly that what you say might be incriminating in some way. Which, given the fact that no person could reasonably be expected to know the whole of the law, and given that lying to a federal agent in the performance of their duties is always illegal, is a pretty safe bet.

Yes. You don't have to talk to cops. But that doesn't make you any less an accessory in this example.

You not incriminating yourself doesn't indemnify yourself.

5th amendment applies to self-incrimination. You can absolutely be compelled to testify against someone else.