I understand the establishment clause as the separation of the state and the church. The United States does not have an established religion and cannot promote any religion over the other. And in addition to that the government cannot promote or prohibit any religious beliefs. The lemon test is a criteria that helps determine if the government has violated the establishment clause.
The US flag is protected by the first amendment because it counts as symbolic speech which is a right in the constitution. Gregory Lee Johnson set fire to a US flag during a protest and was initially charged with desecration of a venerated object. however In 1989 in Texas v Johnson it was decided that burning the flag was a form of symbolic speech.
taking the fifth refers to taking the fifth amendment, in which you have the right to be silent to avoid incriminating your self.