Q1. So, let me see. The conscience is “well formed”, if it agrees with the Church teaching. If there is a difference, then it is poorly formed. Why bother to even referring to conscience?
A1. Conscience is in the heart of man and is available to those that have not learned of the Gospel, for example. Also, culpability for sin will vary depending upon the development. Catechism:
1777 Moral conscience,48 present at the heart of the person, enjoins him at the appropriate moment to do good and to avoid evil. It also judges particular choices, approving those that are good and denouncing those that are evil.49 It bears witness to the authority of truth in reference to the supreme Good to which the human person is drawn, and it welcomes the commandments. When he listens to his conscience, the prudent man can hear God speaking.
Q2. “Ought to have” or “should have” or “required to have” are close synonyms. And the “gravity” of the matter is not something that can be measured on a scale. What is considered a “grave” matter, may or may not be a “grave” matter for someone else.
A1. Yes, as you said, gravity is
not measured on a scale, it is grave because it is serious (not trivial) sin, lacking charity: “A word, deed or desire in opposition to the eternal law” (St. Augustine).
Q3. What is “reasonable diligence”?
A3. A person who is unsure about a Catholic teaching, yet refrains from seeking clarification of those teachings has not exercised reasonable diligence. Catechism
1791 This ignorance can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin."59 In such cases, the person is culpable for the evil he commits. 1793 If - on the contrary - the ignorance is invincible, or the moral subject is not responsible for his erroneous judgment, the evil committed by the person cannot be imputed to him. It remains no less an evil, a privation, a disorder. One must therefore work to correct the errors of moral conscience.
Q4. What is available evidence?
A4. Is has been defined as valid and reliable evidence. For moral matters is it the teaching of faith and morals from the Catholic Church. Revealed truth and magisterial teachings.
Q5. If someone heard the teachings, listened to them, contemplated them and
disagrees with them, and has valid, logical and rational reasons to disagree, then is that “ignorance” invincible?
A5. No, for the truth has been heard. It is not necessary to have a rational understanding. Some truths are mysteries, and a given reasoning is only a approximation. Catechism
1792 Ignorance of Christ and his Gospel, bad example given by others, enslavement to one’s passions, assertion of a mistaken notion of autonomy of conscience, rejection of the Church’s authority and her teaching, lack of conversion and of charity: these can be at the source of errors of judgment in moral conduct.