Prior to Vatican II, Passion Sunday was the 5th Sunday of Lent and Palm Sunday was observed on the subsequent Sunday. 
 
For many centuries, the 5th Sunday of Lent was called “Passion Sunday,” while the Sunday after it was called, “Palm Sunday.” 
 
Currently the Roman Rite denotes Palm Sunday as “Palm Sunday of the Lord’s Passion,” which combines the two observances. 
 
Dom Prosper Gueranger explains in his Liturgical Year the reason why the 5th Sunday of Lent was once called Passion Sunday: 
 
“This Sunday is called Passion Sunday because the Church begins on this day to make the Sufferings of our Redeemer her chief thought. It is called also Judica from the first word of the Introit of the Mass and again Neomania that is the Sunday of the new or the Easter moon, because it always falls after the new moon, which regulates the Feast of Easter Day.” 
 
Passion Sunday also marked the beginning of a special sub-season called Passiontide, which extended up until Holy Saturday. During this time the Church’s liturgy became more somber and a sorrowful mood was reflected in the various practices that occurred in the liturgy. 
 
Palm Sunday was the same celebration as it is today, with the procession of palm branches and the reading of the Passion narrative. 
 
The Church simply saw fit to prepare the hearts of the faithful for the Passion of Jesus during the last two weeks of Lent by calling the 5th Sunday of Lent Passion Sunday. 
 
After Vatican II, the Church decided to combine the two Sundays, removing Passion Sunday from the calendar and adding its name to Palm Sunday. This puts a greater emphasis on Palm Sunday and also reinforces the Passion narrative that is proclaimed on that day.

Jesus of course mediates a relationship with the Father in a way no one else can. No one comes to the Father except through Him. However, in terms of our relationship with him, Jesus has established things and people which help mediate our relationship with him: apostles, evangelists and teachers have roles of service to build up the body of Christ. Faith comes by hearing, hearing by the word of God. Therefore, our relationship with Jesus is mediated by both Scripture and those whom the Lord sends to evangelize us. 
 
You seem to understand "one mediator," in a completely univocal and absolute sense. If so, then you should never ask anyone to pray for you. Neither should you listen to a sermon or even read scripture. For these are things and people which mediate Christ to you in some sense. 
 
Catholics do not hold that the prayers of Saints substitute for Christ's mediation, but rather are subordinate to it, and facilitated by him. For, as Head of the Body the Church, he creates a communion of all the members, allowing, and expecting that all the members of the body assist and support one another. This does not substitute for Christ's mediation, but rather, presupposes it.

It is an abbreviation for Jesus Nazarenus, Rex Judeorum – Jesus of Nazareth, King of the Jews. In Latin the “I” and “J” are usually interchangeable and ancient Latin did not use the “J”. That is why it is INRI not JNRJ. 
 
It was common for the Romans to hang a “titulus” or sign above the crucified to indicate the charges against him. Scripture says that Pilate put the charges in Latin, Greek and Hebrew. 
 
Pilate placed this title above Jesus in scorn and mockery rather than faith. He also likely knew it would irritate the Jewish leaders, which it did (see Jn 19:21). 
 
Yet even in his ridicule, Pilate spoke truth. Jesus is King, not of the Jews only, but of all things.