if you use the dark mask with a ST of 0, you can leave everything else as it was before.
If you use an already rasterized design from PS, use 1-2 px underfilling in step 4 and the screening of the printer.
You cannot raserize an image which is already rasterized.
Yes, the dark 2-paper-system is better washable because of the second paper.
If you use Laser-Light, it is important to use the highest possible pressure for fixing the design with silicone paper after transferring it to the shirt.