I think you missed one point: in CPC "mode 0", pixels are wider, the resolution drops from 320x200 to 160x200 while still occupying the same surface. When the programmer gave you 160x200 to work with, he intended the native pixels - so for example the 8x16 tiles are squares, visually.
If your painting program doesn't allow you to paint in native wide pixels, you double the widths - but it's up to you to remember that you need to double them everywhere : 160*2x200 canvas, 8*2x16 grid, etc.