Does land ownership become legitimate by following the legal procedures in force in the state, which include declaring ownership and having 12 people testify to quiet and continuous possession for more than 10 years, despite doubts about the validity of the original contract concluded by the father 20 years ago, especially after one of the female sellers pardoned him and the male seller confirmed his donation of the land? And are the father and son absolved of responsibility by these procedures, knowing that the son was not present at the first contract?

You are not at fault for benefiting from the land, and you are not required to investigate or inquire about the origin of the contract or the satisfaction of its owners. The fundamental principle is that whatever is in the possession of a Muslim belongs to them, unless there is definitive proof to the contrary. There is no obligation to investigate the source of ownership, whether it is lawful or unlawful. As long as you do not know that the money is unlawful, it is lawful. However, if it is known that a person deals in unlawful matters, then it should be avoided out of piety.