Because it is being implied and stated that a government somehow receives its authority from the Constitution. Authority does not come either frolm an existing law, or from the people, it comes from God.
Again, because it has been stated that the authority by which a government rules comes from the constitution. You are bound to follow any laws passed, and as an individual you do not have the authority to decide whether or not that law is valid (based on your understanding of a pre-existing law). Whether or not a law is valid or not is something for civil authorities and due process to decide, not the individual citizen. The civil authorities are in a position of authority on such matters, the individual is not.
No. What I am saying is that an already existing law is not a source of authority (as has been stated on here).
That may be true, but, nevertheless, we are still bound to follow all laws passed by those civil authorities, unless those laws are contrary to Church teaching and scripture. There are certain, very limited, circumstances where we may overthrow a government (CCC 2243) but other than this we are bound to follow all the law. So even if we have a bad government made up of self-serving people, who disregard previous laws when setting legislation, we are still obliged to follow the laws that are set for us by that government (as they are in a position of God-given authority over us) unless those laws “are contrary to the demands of the moral order, to the fundamental rights of persons or the teachings of the Gospel”.
Yes it does, but the fact that it has a constitution does not mean that, as a Catholic, you are not obliged to follow laws made that appear to be contrary to the constitution (until such time as those laws have been legally overturned through due process).
And if a law is passed that appears to be unconstitutional, then you are still obliged to comply with that law, even if it appears to conflict with your constitution. The civil authorities have the god-given authority to rule, and this authority does not derive either from a constitution, or from the citizens of that country.