How can God's forgiveness of those who commit grave sins be reconciled with the hadiths that exclude those who openly commit sins and those who secretly violate God's prohibitions?

There is no problem with the forgiveness of one who committed a sin secretly or publicly. Whoever committed a sin secretly and did not declare it is closer to Allah's forgiveness. Texts have indicated that Allah may conceal a believing servant's sins and forgive them.

As for the Hadith: "I certainly know peoples from my nation who will come on the Day of Resurrection with good deeds like the white mountains of Tihamah, and Allah, the Mighty and Majestic, will turn them into scattered dust. Thawban said: 'O Messenger of Allah, describe them to us, clarify them for us, so that we do not become like them unknowingly?' He said: 'Indeed, they are your brothers and from your skin, and they take from the night as you take, but they are people who, when they are alone with Allah's prohibitions, violate them.'" This Hadith is not absolute and can be understood in two ways:

1. That these people declare their sins in privacy with other people. So "خلوا" (khalawo - when they are alone) here does not mean "secretly" but "when they have the opportunity." 2. That it applies to those who are reckless and show contempt for Allah's prohibitions in secret, or those who consider sins permissible covertly while outwardly displaying righteousness. This is in contrast to one who sins in secret while fearing Allah and feeling regret, for the latter is not among those who violate Allah's prohibitions.

As for declaring a sin, if it is from the major sins, its perpetrator is under Allah's will if he does not repent, and he is more likely to be held accountable. If the sin is from the minor sins, many deeds expiate it, such as the five daily prayers and fasting Ramadan, just as avoiding major sins expiates evil deeds and leads to Paradise.