You are using pl.move.gravity in c_move, so that it is added to the player position, if the player is stürzing. That means, that the .z value of the sturz has to be scaled completely with time_step. But in your Sturz-branch, you just scale a constant (-0.35) with timestep. If pl.move.gravity is e.g. 64, pl.move.gravity.z will be always more or less ~64, because even on a low fps PL_GRAVITY * time_step will be near 0, and that means, ultimately, that your pl.move.gravity.z will be always framerate-dependent (more or less).

In order to move with c_move downwards in a framerate independent fashion, you need to scale the value of pl.move.gravity with time_step completely (vec_scale).

Since the result is framerate independent then and I guess you want to accelerate downwards, when the player is stürzing, you need to save and refresh the sturzspeed as before:

Code:
pl.move.gravity.z = maxv(pl.move.gravity.z + (PL_GRAVITY * time_step), PL_MAXGRAVITY * time_step);


and when you call c_move, you need to calculate the ***current*** framerate independent sturzrate:

Code:
VECTOR mySturz;
vec_set(&mySturz, pl.move.gravity);
vec_scale(&mySturz, time_step);

c_move(..., &mySturz, ...);