Federal laws do trump state laws in most cases, it is settled. This case is due to the fed's noninterference policy, which they chose for themselves, it's not being forced on them because of some Civil War-era debate.
No, it isn't. The Constitution is the basis of the entire system, and it defines the powers of the Federal government. The 10th amendment says:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.
This simply isn't true. There are areas where federal laws can contravene states laws and areas where they cannot.
Additionally, the federal laws often don't "trump" state laws, but supplement them. Generally speaking (there are exceptions), state law enforcement can only enforce state laws.