[Help changing the colors of a height map to match another]

Essentially I'm working on a map project, and part of it is a height map overlay. I want to change to a new projection but the height map has a different color scheme (off-color file) that isn't the pure white and black as the previous height map for the other projection. When I try to layer the new one using the multiply layer tool, the resulting image is significantly darker, as opposed to the previous projection overlay, which would not darken the image at all except for adding definition to mountains. 

 

So, how can I change the color scheme of one png to match the other? If it's possible?

[Any plug-in to re-size/snap an image into a different frame?]

Like say I have a square picture that I want to re-shape into a triangle. Could it be possible to "paste" the square picture into a triangular or custom shaped frame and have it auto-resize to fit within those dimensions without any cutoffs?

 

[How to fill in a map quicker?]

Ah ok. I will try this out this weekend. Would it be easier to use an easier hue to copy paste then just re-color it at the end? If so, what is the best hue/s to do so? 

[How to fill in a map quicker?]

I use paint.net to make maps, using a blank map template and fill in the states. The problem is once I fill them in, I have to spend a lot of time disconnecting all the internal borders (in one country) from the external borders (separating different countries), then using paint bucket to remove it. This requires me to cut every single line, and if I miss one, every line on the map is turned into the color of the country. This takes a lot of time. 

 

With that in mind, is there a way to tell paint.net to fill in all the internal lines surrounded by the same color? 

 

Then again I know paint.net works wonders but this might be too niche

[How do I make a 2 pixel wide line in an image 1 pixel wide?]

Here. It should be easy as these lines are completely isolated to their own layer, nothing around it. 

[How do I make a 2 pixel wide line in an image 1 pixel wide?]

The title says it all. I have several 2 pixel wide lines in an image that I need to make 1 pixel wide. Going through and either re-tracing or erasing half is very time consuming. Is there some plug-in or built in feature that would automatically thin lines?