The second question does not consider the fact that not only is the Sacrifice of the Mass a sacrifice, it is a Sacrament. A Sacrament, to use the Baltimore Catechism definition, is:
Q. 574. What is a Sacrament?
A. A Sacrament is an outward sign instituted by Christ to give grace.
So, with that in mind, the Eucharist is a Sacrament which gives grace, specifically "Sanctifying Grace." Before we proceed, let us look at what the Baltimore Catechism has to say about the two types of grace:
Q. 459. How many kinds of grace are there?
A. There are two kinds of grace, sanctifying grace and actual grace.
Q. 460. What is the difference between sanctifying grace and actual grace?
A. Sanctifying grace remains with us as long as we are not guilty of mortal sin; and hence, it is often called habitual grace; but actual grace comes to us only when we need its help in doing or avoiding an action, and it remains with us only while we are doing or avoiding the action.
Q. 461. What is sanctifying grace?
A. Sanctifying grace is that grace which makes the soul holy and pleasing to God.
So, the Eucharist is a grace which makes the soul holy and pleasing to God. It remains with us so long as we do not commit a "mortal sin." Let us also look at what the Baltimore Catechism has to say about the difference between mortal and venial sin:
Q. 279. How many kinds of actual sin are there?
A. There are two kinds of actual sin -- mortal and venial.
Q. 280. What is mortal sin?
A. Mortal sin is a grievous offense against the law of God.
Q. 281. Why is this sin called mortal?
A. This sin is called mortal because it deprives us of spiritual life, which is sanctifying grace, and brings everlasting death and damnation on the soul.
Q. 282. How many things are necessary to make a sin mortal?
A. To make a sin mortal, three things are necessary: 1.a grievous matter, sufficient reflection, and full consent of the will.
Q. 290. What is venial sin?
A. Venial sin is a slight offense against the law of God in matters of less importance, or in matters of great importance it is an offense committed without sufficient reflection or full consent of the will.
Back to the subject of the Eucharist, it provides Sanctifying Grace which remains with a person so long as they do not fall into mortal sin. Being that it provides Grace, it also strengthens and increases our holiness as we grow in Christ - or the process of "theosis" (becoming more and more like God). So when we partake in the Eucharist many, many times it is not because we think Jesus Work was not finished - but because He increases in us with each time we receive Him in the Eucharist.
So, on to question 3... How can Jesus be the Real Presence when He has departed and gone to the Father? The problem our questioner has here is a myopic view of Scripture. He/she appears to focus upon a single verse from Scripture while ignoring others... for our response to this question, let us present the questioner with Matthew 28:20: "...and, lo, I am with you always, even unto the end of the world. Amen." So, is He with us - or not?