Hi, Chris.
That shortcode’s attributes come from /wp-content/plugins/events-calendar-pro/src/Tribe/Shortcodes/Events_List.php
And then it runs PRO’s Event List Widget output, which is from /wp-content/plugins/events-calendar-pro/src/Tribe/Advanced_List_Widget.php
As far as the template that gets used, that’s at /wp-content/plugins/events-calendar-pro/src/views/pro/widgets/list-widget.php, which is able to be overridden via our Themer’s Guide.
So it sounds like you’re on the right track going that route but may have your override file in the wrong place or maybe copied the free version’s instead of the PRO version’s list-widget.php
Please let me know if I can be of further help here.