Hey Lode
Sorry for answering so late. I did not received any message about your post.
Usually the uninstall routine should remove all table entries (which means all custom widgets will be lost, but the plugin itself can be used (again)).
If this does not work try to delete the corresponding table entries manualy (table: wp_options, option_id: mcw_*). Can you send me the code of the last widget you tried to add?
You can send me the screenshots to my mail-address whoch is available in the impressum on https://www.janek-niefeldt.de