I'd be glad of further clarification of the last paragraph; particularly in light of the Holy Father's words to the assembled British Lords and Members of Parliament in Westminster Hall, "[t]he Catholic tradition maintains that the objective norms governing right action are accessible to reason, prescinding from the content of revelation."
If, say, the sense of omission entails willfully ignoring what Church teaching one has heard contrary to his inclination, then I can square your last implication with the Pope's words. Again, if it means a cavalier incurious attitude to what the Church might say on some moral question, I can understand that this would be reckless for a Catholic casuist. But if, for instance, you had been raised on Confucius and Mao's red book, apart from the inherent dischord I should excuse your poor soul for omiting questions about Catholic moral teaching.
To clarify, when I wrote "if, a priori, I omit from my determination [of the moral nature of an act] the teaching of the Church," I had in mind something like, "if I, who profess the Catholic faith, decide to ignore what the Catholic Church teaches about the morality of the act." Or, in some guy's words, "willfully ignoring what Church teaching one has heard contrary to his inclination."
The point being, again, that willfully ignoring what Church teaching one has heard contrary to his inclination is contrary to Church teaching on conscience.
The Pope's point about objective norms governing right action being accessible to reason touches on the distinction between accessible to reason in principle and accessed by reason in practice, which St. Thomas mentions in the first article of the Summa Theologiae, on the necessity of Revelation:
Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors.
Granted, objective norms governing right action is not nearly the challenge to human reason that truths about God are, but the same principle applies.
When a person goes to determine whether an act is morally good, he will often find that there are mutually exclusive goods involved. Right reason dictates he not sacrifice a greater good for a lesser good, and I'd guess most grown-ups can see that objective norm clearly enough. The challenge comes in properly ordering the goods relative to one another, and it is in performing this ordering that Church teaching can be particularly helpful.
Someone who does not use Church teaching in a particular case -- be it through ill will, culpable ignorance, or non-culpable ignorance -- may still reach the correct conclusion about the moral nature of the act. A non-culpably ignorant actor may even apply the correct process -- that is, apply the objective norms governing right action as they are apprehended by the actor's human reason. But, as I said before, those of ill will or culpable ignorance are not performing the act of conscience correctly.