Dear Henrik,

You said:

• Should we be bowing at the Penitential Rite in the OF?

The rubric in the current Roman Missal (for OF Masses) says that the priest and the people say the words through my fault, through my fault, through my most grievous fault while striking their breast, so that is what's required. Of course, when people strike their breast, a bow to some degree is almost inevitable.

You said:
Also, I have seen that when the altar servers in the EF say the Confiteor or Kyrie, they kneel.

• When saying the Confiteor, why do the altar servers kneel but the Priests stand and bow?

The Missal gives instructions on what is to be done but does not give explanations as to why things are done a certain way and not another so any discussion of why the priest approaches standing and bows, while the servers remain kneeling, is a matter of interpretation and opinion, so this is my opinion:

It is fitting that the priest recites the Confiteor standing (and bowing as directed) since he has just finished reciting Psalm 42 (Introibo ad altare Dei, I will go in, to the altar of God). He acts as though he has taken on the role of Christ and received permission to enter the sanctuary and pray while standing.  But that is an opinion from me, not an official statement.

You said:

• Is sitting also a penitential posture?

The liturgical book Ceremonial of Bishops has a discussion of signs of reverence (starting at paragraph 68) and lists bowing, genuflection, kneeling, incensation, and kissing, with a brief description of when they are used, but there are others it does not mention: e.g., prostration. It doesn't designate any of these specifically as penitential gestures.

You said:

• Also, Sunday is not a penitential day, so why do we say penitential prayers that day?

Every Sunday is a feast day recalling the Resurrection of Christ, and it's not a penitential day, in the sense that we don't have to observe our personal Lenten sacrifices on Sunday.

That said, the human condition remains on Sunday, what it is every other day, and it is perfectly legitimate, for example, to repent of sins on a Sunday and go to Confession on that day, as many parishes offer Confession before Mass.

In addition, the Mass itself always includes the element of atonement, in that the atoning sacrifice of Christ becomes present, so it is fitting that the Mass includes its penitential elements whenever it is offered.

I hope this helps,

Andrew