Yup, the fuel use rate is woefully inadequate for it to be a fusion rocket that functions as a rocket. The rocket equation is a harsh mistress.
MTU explanation was/is the m-drive uses gravitic technology to lower the inertial mass of the ship - since inertial mass and gravitational mass are at the moment proving to be the same thing. If grav modules can reduce gravitic mass for the purposes of being attracted to a planet then they should also reduce inertial mass. Since this is a field around the ship it explains why it is volume based. The inertial mass is reduced to such an extent that a plasma drive, ion engine or 'fusion' advancement of the plasma drive can accelerate the ship up to 6g (now 9g). You can't push it a higher acceleration because the field will collapse.
Strangely enough nuclear damper technology would make this more efficient at TL12+ since you can enhance the fusion reaction and decrease the harmful effects of the fusion exhaust.
If the gravitic bit of the m-drive is damaged then acceleration is reduced, lose all your gravitics and your engine thrust will be reduced to micro-g values or worse.