Is a written acknowledgment from the husband that if he returns to committing the major sins mentioned, he will be considered irrevocably divorced by three pronouncements (talaq bain bainunah kubra) legally valid, and does divorce actually occur thereby?

The husband writing a conditional divorce paper may or may not be effective, as there is much jurisprudential disagreement on this matter. If there is hope for the husband's reformation and the disobedience does not harm the wife and children, then patience and remaining together are preferable, especially if the husband repents and returns, as this indicates good in him. It is recommended to make abundant supplications and seek forgiveness.