Yes, it was well known in the neghborhood that Vito killed Fanucci, but not well known to the police. The book explains this in far more detail, however, we will stick to the movie. Remember, Vito went to Fanucci's house over the rooftops. That means that nobody saw him enter Fanucci's house. He was also not known to the cops, being a law-abiding citizen until his joblessness "drove" him to commit the dress robbery with Clemenza and Tessio.

As for Michael and the McCluskey/Sollozzo murders, the book again explains how this occurred. Vito got someone else to confess to the murders. This man was facing the death penalty anyway for a murder he had committed and confessed to. Vito promised to financially support his family in exchange for his confession, and Michael was brought home. I don't think any of this is ever explained in the movie.