The scenario: A Soyuz-MS spacecraft is on-orbit and ready for its deorbit burn. The crew then discover that there is a fault in the propulsion system and the deorbit engine can't fire. The craft is stable, in the correct orbit, and (apart from a non-functional engine) is fully intact. Is there any way the crew can return safely to Earth?
Nature of the failure (choose one):
Either the N2O4 or UDMH feedlines (you pick which) are fully blocked. Could the deorbit engine be used as a cold gas thruster? If so, would it have sufficient thrust?
Total failure of the N2O4/UDMH subsystem; it's intact but completely non-functional. Do the other thrusters (such as the H2O2 thrusters on descent module) have sufficient thrust to deorbit the craft? Are there any outside-of-the-box solutions?
Docking with a rescue vehicle/tug is specifically excluded.