How can common believers in the Roman Catholic Church be sure that the Church's decisions are correct and not erroneous?

Many Protestants will make the claim that although the Bible was written by various fallible men, that God guided these different men perfectly to create the Bible. However they reject the similar claim that God could guide the Popes in certain circumstances to be free of error.

The answer to your question, "How can they be sure its correct", is that its a matter of faith.

The second part of your question states that these people were heretics. People can err on a matter of faith and still be in the Church. Its only when they obstinately reject a defined truth of faith do they become formal heretics who automatically incur excommunication from the Church.

Pope Pius XII, Mystici Corporis Christi (# 23), June 29, 1943: “For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy.”

Briefly, not all papal decisions are ex cathedra, or spoken from the chair. When he delivers statements in this mode, they are matter of doctrine and dogma, and this has seldom happened in history. Nothing every stated ex cathedra has ever been reneged, and if you read what these things are, you'd realize there is no possibility of reneging them without essentially contradicting Christianity.

Now does this mean that all other teachings are optional? Certainly not. The difference is that the other teachings are knowable without revelation, and can be known through the use of reason or reason augmented by faith. Because scripture is interpreted with the help of tradition (which was abandoned by Protestants, although the Orthodox situation is unclear), the Church is the best qualified to interpret it. The Church is not settled on all matters of Scripture, which is why theologians still debate certain points, but these don't have any larger effect. You have to be careful to distinguish between canon law, unsettled Scriptural matters, and infallible dogma. If you don't distinguish, you become the kind of Protestant who thinks Catholics worship saints, the pope, relics, and so on.

With respect to condom and contraceptive, the pope has not made their use acceptable. This is a wild misinterpretation that the media has propagated (and the media in Anglo-America is notoriously anti-RCC). The Church has always, does, and always will maintain that contraception is evil. The pope, in a rather insignificant explanation that a depraved media fixated on, is merely saying that if someone is a male prostitute now (which is an evil state of affairs), then a move in the right direction may involve condom use as part of an incremental shift from a greater evil to a lesser evil. By using condoms, he is reducing other evils (the act and everything else remains just as depraved) such as the rate of HIV transmission (assuming effectiveness). He's not saying it's okay. In fact, if one is a male prostitute, one should stop being one immediately. The pope is merely saying that in such limited cases, a condom can be seen as a step in the right direction. Not that it's a very big move, or a noble move, or one to write home about. He's giving a theoretical account of it. But in reality, his position is no change at all. Homosexual acts are intrinsically disordered and are not made less evil themselves through condom use. Condom use may merely reduce an evil side effect of disease transmission.

So be very careful about what the Church is really saying. You make the same mistake with the aforementioned councils. Those who engage in attacks on the papacy are usually unwilling to really learn about the Catholic Church in depth, and prefer to believe a rather erroneous account of it. There are over 40,000 Protestant denominations because every Tom, Dick, and Harry, plus Joe the Plumber decide, with no education in these matters, what Scripture means for them. Not what it really means, but what they want to read into it.