I started off doing the same thing you did. Mixing powder with acetone.
It got me my first dark dark colors with dyes. And I used brushes, and the outlines aren't from contact paper or anything. I used a wax crayon to color in parts that I did not want to dye. It worked ok for a bit.
That was the first 2 dyes I did. The DGR one is blue, and the death star one is black. The black is more of a dark dark brown like what you're describing.
As I did a few more dyes, I found that the brush was just annoying. It just didnt leave the solid colors that I was aiming for. That's when I took up the pan and started with the heat.
1) Yes, I actually have an Xacto blade that's got duel blades around 1/4" apart. It's labeled as "X-ACTO 7747 Foamboard Cutter, Foam Core Cutter"
2)Only way I've done thin thin lines is to just go the Contact paper/vinyl route. ive got some dye work with paper thin lines, but just realize that over time that thin line will fade and blend into the plastic... and not be so clean.