MacOS + Firefox. The image processing relies on the web browser's Canvas API, so depending on your setup, there could be differences which might cause it not to work on the Switch. At the end of the day, I don't have control over that part.
It should work if you set the Uae4All2 nro file as the Core Path, and then set the path to the game file as ROM Path.
If that doesn't automatically launch the game, then it's possible that Uae4All2 is not checking the argv when launching to boot straight into a game, in which case you'd need to...
Can you share the image that you are trying to use? IIRC there's a max file size limit that the Switch will accept (128kb?) and if the image is larger than that then it will just show the question mark. Maybe I can tweak the quality setting to ensure the size is within the allowed range.
I...
I haven't tried it, but it depends on if that standalone melonds supports specifying the ROM path as a parameter. Give it a try and let us know!
If it does work, we should add a preset for it to the dropdown.