The teaching of the catholic church can never trump the simple direct easy to be understood teachings of The Lord Jesus in the gospels. Why add to and complicate what the Master the Lord Jesus has given to us ?? The Holy Ghost will teach you all things (give you understanding/ insight) of what is written there. Why not take advantage of the fact that God Himself lives within your spirit and give Him a chance to give you insight and revelation into the wonderful scriptures ???? You were confirmed were you not ?? Did you not receive the Holy Ghost then ????
By what authority do you claim that your interpretation of Scripture is true? The teaching of the Catholic Church does not trump, but proclaim the teachings of Our Lord, by His authority that He conferred to His Church.
In any event, the Pope was Catholic. Why do you think it unreasonable that Catholic Church teaching to pray for his soul be followed by other faithful Catholics? You have no dog in this fight.
The catholic (NB lowercase "c") Church preceded the gospels. Therefore, the sacred truths and the practices and the dogma are foundational to the written word. This written word was provided to the Church already founded in the teachings and traditions of Christ.