Any way to modify gravity direction for actor?

I can help… I am a kind of game physics specialist and studied Physix a lot for Unity and other engines.

AddForce adds a perennial force in every physics update (i.e. gravity or rocket thrust…) , while addimpulse does it just once, as if firing a bullet.

Accelleration is calculated as A = F/M where F is force and M is body mass, so no need to use deltatime in either case, since Physix will take care of interpolating velocities due to accelleration

AddTorque works similarly but for angular (rotational) forces, the angular accelleration (a) is calculated similarly to forces but using the inertia tensor on each axis
so a = T/I , also here ignore deltatime