a) Is there a way to create an overview/dashboard of webinars? Is there a way to centrally access them so they are not limited to the person who set the webinar up?
Webinar in Teams is working like normal meetings, which is stored as an event in the organizer's mailbox.
Thus to me there isn't a built-in method to centrally manage all the webinars.
b) Is it possible to set up an automated flow using Power Automate (or other app) for Teams external webinars?
Yes it should be possible.
Here is a thread which may be helpful for your reference: Specific meeting as a trigger to send out an Email
For detailed guide, I would recommend posting in the Power Automate support forum to get better support.
c) Can I set up a webinar on behalf of someone else?
Would you clarify on set up a webinar on behalf of someone else?
Did you mean creating the webinar via your Teams client while making another person as the actual organizer of this webinar?
If yes, you can add others as the presenter of this webinar, or assign them as co-organizers of the webinar.
Here are two links talking about the roles and permissions in Teams meeting:
Add co-organizers to a meeting in Teams
Roles in a Teams meeting
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