About the zoom issue:
I also thought about it; that maybe it is OS-related, because I'm using Arch Linux. I know that there is no version for that particular type of Linux; I downloaded the DEB-package and extracted the files using 7Zip, and find out which files needed. I copied the 'ditherista' binary file and the 'libdither.so' file into a folder and it worked. So, maybe that mess up with the mouse. I could use Wine-emulator to run the Windows version, but that is not really a great option. I will try out and see if this reversed-zoom issue still happens.
I didn't know about that how Amiga HAM works and that it is not that simple to implement it. Accuracy is important, at least to me, so loading palette in a image-editing software is not an option.
About those 32K and 64K colors. I think you misunderstood why I really like to see those. It's not about that, that with more color I can get better results. It is about emulating the early 90's DOS era look when video cards can't display 24bit True-color, only 15/16bit colors. It is more about emulating that look using different dithering, to achive that kinda oldschool aesthetics. How fast or slow will be the conversion is not really a problem to me; obvioulsy because of the whole retro-thing I don't work with really large images.
Plus/4 palette:
I use the Yape emulator which is an emulator for Commodore 264 series computers. IMO this emulator has the most accurate Plus/4 colors. http://yape.homeserver.hu
There is an SDL port of it (old version but the colors looks the same), you can check the source files and find out what palette it use. https://github.com/calmopyrin/yapesdl
The VICE emulator is more of an all-in-one Commodore emulator, and this one can load palette files, and also include different palettes! It is includes the palette from YAPE; the PAL palette looks the most accurate IMO. Other palette-files (or the built-in palette) looks weird; they are too much pinkish.
https://vice-emu.sourceforge.io/
If you grab the 'vice-3.10.tar.gz' and extract it, in the /data/PLUS4/ folder you can find the 'yape-pal.vpl' palette file. That is the good one. This palette extracted from a screenshot made with YAPE. I don't know how accurate palette you can get from a screenshot. But it is defenitely easy to use, it just a text file with RGB values.
Better to check the YAPE sources to be sure.