They wrote the New Testament based upon the Word of God

(continued from yesterday)

Lutero:

You do realize that your interpretation is not the same as the RC leaders who claim they alone have the authority to interpret God's word.

Show me the Catholic Doctrine which says that the Catholic Church ALONE interprets Scripture. I want to see it from a Catholic source.

So if the apostles interpreted Scripture

They didn't. They wrote the New Testament based upon the Word of God transmitted to them by the Teaching of Jesus Christ. Jesus Christ did not write a word of Scripture. He spoke to them.

and their interpretations are God-breathed,

Their Teachings are God breathed. Their prophecies are God breathed. Their speech is God breathed.
2 Peter 1:19-21

King James Version (KJV)
19 We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
20 Knowing this first, that no prophecy of the scripture is of any private interpretation.
21 For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.

why aren't the interpretations of the Roman Catholic pontiff and magisterium also God-breathed

They are God breathed. They are the Word of God. Scripture and Tradition are both the Word of God.

and therefore Scripture as well?

Here is the Catholic Teaching:
97 "Sacred Tradition and Sacred Scripture make up a single sacred deposit of the Word of God" (DV 10) in which, as in a mirror, the pilgrim Church contemplates God, the source of all her riches.