Hi,
Thanks for the interest in our plugins. I can answer your questions.
You do not need Pro for The Eventbrite Tickets to work. It does require the core version of the Events Calendar found for free on WordPress.org: https://wordpress.org/plugins/the-events-calendar/
Which relates to your second point, please try The Events Calendar first with your theme, if it works then Eventbrite Tickets will work as that is what is used to display the Eventbrite Ticket’s Events.
The license is not required for the plugin to work, just for the automatic updates. But we have this guide to moving it:
https://theeventscalendar.com/knowledgebase/moving-your-license-keys/
Let me know if you have any follow up questions.
Thanks