Here are some first experiences with your GUI application.
I tested it in High Sierra, I put your GUI in the folder where the BasiliskII application and the old GUI reside, and I started with copying my existing ~/.basilisk_ii_prefs to ~/.basilisk_ii_prefs_test.
1. Closing the window does not quit the application
I am used to dismiss such one-window applications by clicking the red button. Most such applications will then quit. Your GUI app continues running when its window is closed with no way to bring the window back.
2. Lines in the prefs file are duplicated
Saving a change in the GUI should only change the related line in the prefs file or replace the content with the new settings, but often lines are duplicated or even all lines are duplicated as if the new settings are appended to the existing ones. After only a few changes in the settings my prefs file had grown and looked like this: http://ronaldpr.home.xs4all.nl/basilisk ... ntries.txt
3. Volumes list, Finder icons
I am not sure why you use Finder icons before each volume in the list, regardless what kind of volume it is. It seems to indicate a compatible startup volume, but it is used for any volume.
4. Volumes list, "Kind"
There is a column "Kind" that indicates that the volume is a disk image. Isn't that superfluous information? Aren't all volumes in the list disk images?
5. Volumes list, "Not Found"
If a volume is not found, this is displayed in the Kind column. That is useful information, but the further implementation is problematic:
6. Files not found will be removed
When settings are saved, files that are not found will be removed from prefs.
For me that is a problem. I am in the habit of moving files out of the way in the hosts Finder when I do not need them and move them back when I need them, That way I can choose which disk images will be mounted in BasiliskII without the need to change settings. I do not know if many other users handle disk images with BasiliskII or SheepShaver the same way. Both applications will simply ignore incorrect paths to disk images.
Another problem for me is that I often use relative paths (file names only) for files that are in the same folder as the BasiliskII application. Those will also be removed when I save changed settings. The old GUI does not change such entries.
7. Incorrect removal of not found disk images
When not found disk images are removed from the prefs file, only the path is removed. The "disk" entry remains, causing persistent "Not Found" volumes in the list. The whole line should be removed.
8. Incorrect reading and writing of values for CPU type and FPU.
I have not extensively tested this, but I seem to be unable to set all combinations correctly.
These five settings in the GUI
CPU 68020 with FPU
CPU 68030 with FPU
should we written in prefs as:
9. Does not create a prefs file when none exists
When launched, the GUI app should create a prefs file when none exists. It could be an empty file, but it could also be a file with some default settings. More recent (2011) versions of the old GUI create this file with some preconfigured settings: http://ronaldpr.home.xs4all.nl/basilisk ... refs_1.txt
Personally I would choose other values to be the default for some of these settings but that can be discussed later.
If these settings are copied to ~/.basilisk_ii_prefs, your GUI app will crash at launch. Not sure which lines cause the problem. Maybe it is because items are in a different order.
BasiliskII itself also creates a default prefs file when none exists, but those settings clearly date from years ago. Apparently no one bothered to change those defaults in newer BasiliskII versions: http://ronaldpr.home.xs4all.nl/basilisk ... refs_2.txt
Also with these settings in ~/.basilisk_ii_prefs your GUI app will crash.
I did not yet try every setting, I did not yet try presets, and I have some thoughts about the interface, but that can be discussed later.