Ranks change arbitrarily: Tuvok's rank insignia changes between Lieutenant and Lt Commander several times. Tom Paris' rank insignia changes between Lieutenant JG and Lieutenant several times.
Lt. Torres is shown ejecting the warp core with just a few console commands in order to prevent Voyager from entering the nebula, which is an incorrect procedure and would not stop the ship. In previous Star Trek series, and later episodes of this series, it is established that it takes the voice authorization, plus the command codes, of the chief engineer, or another senior officer, to activate the emergency warp core ejection system; plus for safety reasons the warp core cannot be ejected until all personnel have cleared engineering.
Also Voyager was entering the nebula at one-half impulse, a starship cannot safely traverse a nebula at warp. Impulse power is generated by fusion reactors located behind each impulse engine, ejecting the warp core would do nothing to stop the impulse engines. The ship's main power comes from the warp core, with it ejected the ship is stuck at sublight velocity and running on auxiliary power, which is also generated by fusion reactors, it weakens their shields and phasers but certainly does not stop the ship dead in space.
Also Voyager was entering the nebula at one-half impulse, a starship cannot safely traverse a nebula at warp. Impulse power is generated by fusion reactors located behind each impulse engine, ejecting the warp core would do nothing to stop the impulse engines. The ship's main power comes from the warp core, with it ejected the ship is stuck at sublight velocity and running on auxiliary power, which is also generated by fusion reactors, it weakens their shields and phasers but certainly does not stop the ship dead in space.