The first of the two freedoms is the establishment clause which prevents the establishment of religion so no law can pertain to a establishment of religion 

The Supreme Court case Texas v Johnson ruled that flag burning represents symbolic speech which is protected yes by the first amendment, ruling it protected under the constitution 

I plead the fifth means they’re pleading their fifth amendment basically invoking it so they don’t have to testify against themselves and risk incriminating themselves basically meaning they have right to stay silent