Is it true that an 'urfi marriage is simultaneously Islamically valid and forbidden? What is the difference between the two? And does the prohibition cease if the wife waives some of her rights, such as inheritance and maintenance, and refrains from having children?

If the conditions and pillars of marriage are met, then it is valid, even if it is not documented. This is a form of customary marriage (zawaj urfi). However, documenting marriage in our time is obligatory to preserve the rights of the spouses and children. If it is not documented, then it is unlawful due to the abandonment of this obligation. Thus, the marriage is valid, but the one who proceeds with it without documentation is sinful. This is the meaning of something being valid yet unlawful, unlike marriage without a guardian (wali), which is unlawful and invalid. The opinion that documentation is obligatory is clear, and the benefit in it is great and evident, while the harms of abandoning and neglecting it are well-known and widespread, especially in times when denial, repudiation, and the loss of rights have become prevalent. Some personal status laws have obliged courts not to hear or acknowledge claims of marriage unless an official document is presented. In the past, people did not need this documentation due due to the scarcity of corruption.