It is permissible to intercede for a Muslim to help them to fulfill a need or to achieve something that they desire provided that doing so will not lead to sin and will not harm anyone. A proof for this is a Hadith which was related by
Muslim on the authority of
Abu Musa (may Allah be pleased with him) who said that when a person would ask the Prophet (peace be upon him) to help him with something that he needed, the Prophet (peace be upon him) would look at people in his presence and say: (Part No. 25; Page No. 392)
Help and recommend him and you will receive the reward for it; and Allah will bring about what He wills through His Prophet's (peace be upon him) tongue.
However, it is not permissible to intercede for something that involves a sin or comprises inflicting any harm against others as in the cases which are mentioned in the question. In other words, a person will be considered sinful if they intercede in separating someone and his wife, or marrying a woman to someone who does not suit her, who does not fulfill his religious duties, or that she does not agree to marry to; or to propose to a woman to whom another man has already proposed.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.