First of all, the "Calendar widget" is turned into "ToDo Agenda" a couple of years ago, and it has the new GitHub repository and the Issues list: https://github.com/andstatus/todoagenda
Widget IDs are assigned not by our widget, but by an Android System (maybe by a launcher app, I'm not sure actually...). This is why you couldn't reset them by uninstalling the widget app.
How can I reset the widget ID? uninstall-reinstall keep counting up the widget ID.