Who is the guardian (wali) in marriage, and is it permissible for a woman to appoint a Muslim who is not a mahram or a family friend as guardian?

If a girl asks to marry a suitable man and both desire each other, her guardian must marry her off; otherwise, he is sinful and considered to be obstructing her marriage (عاضلاً), and guardianship passes from him. Scholars have differed on whether guardianship transfers to the next distant guardian or to the ruler (Sultan), but they agreed that it transfers to the ruler if all guardians refuse to marry her off. They based this on the Hadith (definition): "The ruler is the guardian of whoever has no guardian." If there is no Muslim ruler or his deputy, a just Muslim may marry her off with her permission.