First off, simple question...Do you go by Charlie or Charles?
I understand what you are doing with the tweaking. Currently, the program speed is not problematic at all so I think you can go for increasing sub division and other tweaks at the expense of speed. Waiting for items to render is much preferable to not having clean conversions. Maybe, at some point, putting in a time estimate for render would keep folks from thinking that the program has locked up.
I spotted the invert option issue and was going to add it to the issues list. No need now.
This next suggestion is from my perspective, others may have a different opinion. What you described with the way you tweaked the picture is nice but I am a just a basic Photoshop user. I know enough to be really dangerous. I had already played with the contrast/brightness but the gradient thing is outside my knowledge level at the moment. The more that you can do to make PCNC handle the adjustments without having to use 3rd party software is better. To make the product better and adoption by users easier, they will want to load, adjust, output and carve.
Thanks for the hard work. Looking forward to getting my hands on the new version.