In the specific case you're talking about, always.
IANAL, but my understanding is that evidence obtained from an invalidated warrant is disallowed, unless the court finds that it was inevitable that the state would have discovered the evidence.
Nope. If the government fooled the magistrate, it is disallowed. However if the magistrate has failed to decode the law correctly and issued an unlawful or unconstitutional warrant, the evidence obtained by the warrant can be used in court.