1) Are you looking to erase the color, or just replace the color? How much tolerance do you want? Like strictly that color, or similar colors? Because there are a lot of "blue" variations in that image.
To erase a single color you can use the "Background Eraser" this eraser works by sampling colors, adjusting the tolerance, and then erasing those colors. It usually does this dynamically to detect edges and things. However, you can tell it to sample continuously, or only once. If you tell it to sample only once, then it will sample the color under the center of the brush, and for the duration of the drag will erase only that color, within the tolerance level you set.
If you want it to be strictly one color set the tolerance to 0.
If you would like to replace the color, you can use the color replacer brush.
I have videos on these tools that might help
Background Eraser Part 1-
https://www.youtube.com/watch?v=pRgHaVMxgEY
Background Eraser Part 2 -
https://www.youtube.com/watch?v=iHfk6VIJE1Q
Color Replace -
https://www.youtube.com/watch?v=B9DqLjA57_o
2) Not sure what you mean?
3) Yes it is.
Install the plugin, find the installation folder after it's installed so that you know where the plugins are.
Open PSP
File -> Preferences -> File Locations
Select "plugins" on the left
Add a new location to look for plugins, browse to where the nvidia plugins are installed.
Save
Restart PSP