Why is there no Gospel account of Jesus’ meeting with his mother right after his resurrection or in the 40 days before his ascension?
Acts of the Apostles 1:14 says that Mary joined the apostles in the upper room immediately after his ascension. The suggestion is that she was with them prior to that event. The Gospels are not biographies as such but rather faith accounts to encourage people to become disciples of Jesus. If we accept them for what they say about themselves (e.g., Jn 20:30–31 and Jn 21:24–25), then the difficulty you raise can be seen in its proper context.
If Jesus knew all along that Judas would betray him (Mt 26:20–25) and if Jesus came for sinners, doesn’t this sound as though Judas had no choice? Was this preordained?
No, it was not preordained. Although Judas had a genuine choice, we cannot conclude that he was condemned to hell for all eternity. God is not subject to past/present/future the way that humans are. This renders all talk about predestination impossible. After the fact, people may describe an event as preordained, but that is not true because it would force God to act within the bounds of human, chronological time.
Why in Christian art is God the Father often presented as an extremely old man, perhaps over 100 years old?
The Book of Daniel’s description of “the Ancient of Days” (7:9) is probably responsible for this tendency. Regarding God and human time, see the Q&A immediately before this one.
What is the difference between tradition and the magisterium?
Tradition can cover many questions and activities. For example: Does human life begin at conception? Should statues in Catholic churches be covered in the final weeks of Lent?
The Church’s magisterium (teaching authority) weighs in on some issues (human life begins at conception and should be respected) and leaves other things to local decisions (covering statues during Lent). By regularly sharing in the Eucharist, believers develop a sense for which issues need to be resolved at what level of the Church.