Is it permissible for the guardian (ruler), due to a necessity he perceives, to determine the return on bank projects based on a feasibility study, by analogy with the permission granted by some scholars for price-fixing, despite the existence of a sound prophetic tradition (hadith) prohibiting price-fixing?

Equating usurious interest with price control is a flawed analogy, due to the difference between them: Usury is forbidden by the unanimous agreement of the Ummah and by texts that are definitive in authenticity and meaning, whereas price control is a matter of scholarly disagreement, and the Prophet's (peace be upon him) prohibition of it is not definitive in its indication of prohibition. Furthermore, usury contains no public benefit, unlike price control, which may be necessitated by public interest.