It needs to make a polished groove or smooth surface-divot. Definitely use colored or black plastic, to promote surface-absorption. The real key is to eliminate jaggies. Either your steppers need micron resolution or better, or the laser-sweeping needs adjustable radius (smooth curves, but stepped depth.)

I tell people, if you could play the hologram on a record turntable, the scratches must be silent. No micro-wiggles. The must behave as long, curved mirrors.

IIRC, no, because the edge that makes isn't sharp enough.

it may if you focus it properly. Acrylic melts really well with IR lasers this may be easier they are cheap.