a. Unfortunately, Adding the folder didn't work. Somehow the new layout didn't appear in Keyboard Preferences. Even though I restarted.
I forgot the initial discussion: what folder did you add? And where?
b. "But I managed to install it by simply saving the keyboard layout into my library,”
This is a wrong decision, while working with a keylayout, which is in an interim, unfinished state, you should NEVER save in directly in the library.
While in its unfinished state, save the keylayout file anywhere outside the /Library, e.g. on your desktop or in the Documents folder.
c. then installing manually from there. (not exporting as a disc)”
Yes, one may simply drag-and-drop a keylayout file to its destination: /Library/Keyboard Layouts, but this should be done when all ready, i.e. when you consider your work done and ready for testing/first use. If something goes wrong, go back to your keylayout and improve it. This is why it is recommend to move to /Library/Keyboard Layouts a copy, and let the keylayout file for some time in its interim location. To move a copy, press Option/Alt file while dragging the file to its location, this will create a copy of it, while the original remains untouched.