The establishment clause is simply the restriction of having any type of religious establishment being sponsored by the state by prohibiting any type of law in relation to it. In relation to this, the lemon test also practices no correlation with religion, thus having a secular legislative purpose.
The burning of the flag is protected by the first amendment because the supreme court decided that in the court case “Texas V Johnson”, that the burning of the flag is a form of symbolic speech.
When someone says that they are “taking the fifth” or “plead the fifth”, they are simply saying that they have the right to remain silent.