A - First, let us define some terms.
Communion is a gift from God. It is not created by man. Until our communion with God and each other is consummated completely in heaven, the Sacramental communion with others and God is the extension of this relationship while on earth. This Sacramental communion echoes the visible and invisible dimension of union with the Church and Christ. Because Christ was both spirit and body, we are to be united to him in both ways. This unity is therefore Sacramental.
Remember that when we are talking about unity with the Church, we are talking about unity with Christ's body. There is one Christ and one truth. The Father spoke one Word of Revelation in the form of the person of Jesus Christ. Therefore, we must see the teachings of the Church as a whole Word of Truth, spoken by the Father to reveal Himself to us. It isn't as if there are some truths that we get to choose from and throw out the rest. There is one truth about God and His Church.
Therefore, communion is about the interpersonal relationships within the Church and the bond of unity. We cannot forget that communion implies the human and divine dimensions. This means that communion is not just a work of man, but primarily a work of God and our participation in his communion of Father, Son and Spirit.
The act of receiving someone into the Church fully (one who is already baptized) includes a statement of faith in which they profess the following:
I believe and profess all that the holy Catholic Church believes, teaches, and proclaims to be revealed by God.It is a statement of faith in the one truth of God, as revealed through Christ and entrusted to the Church. To be in full communion is to hold the one truth of Christ as true in all it's forms.
While the conscience should always be followed, we must always form it according to the teachings of the Church. The Catechism talks about the Conscience in paragraphs 1776-1802. This is a short section that talks about the formation and following of the conscience that I highly recommend. As it states in paragraph 1785 - the authoritative teacher of the conscience is the Church. So, to disagree with the Church is to fail to follow the correct teacher.
There is more on Communion with the Church in the following documents:
On Hans Kung, the question becomes - can a loyal Catholic theologian publicly dissent from Church teaching and remain in full communion? The answer; it depends, but with some serious conditions to that answer.
If the Church has taught something infallibly or made a "definitive statement" on matters of faith or morals (in other words, something that is closely related to revelation and is to be "firmly accepted and held" - examples include contraception and all-male priesthood) then no, one cannot dissent publicly.
If however the teaching is on ordinary teaching on faith/morals (pastoral judgements and applications of certain principles) or prudential teachon on disciplinary matters, then one can dissent with some conditions. Those would include; informed disagreement after sincere effor of submission of intellect and will or external conformity not possible, duty to inform magisterium of problems raised by teaching, patient, humble, open-minded dialogue with magisterium (must remain open to being taught), and finally must not use a pressure tactic or journalistic approach to try and change a teaching (e.g., petitions, etc)
For more on the issue of communion as it relates to Theologians and dissent, you can read the Congregation For the Doctrine of the Faith's letter about this issue - Donum Veritatis.