Hello,

and thank you!

Is there any option in the configuration panel to send an e-mail notification to an event's creator that the event has been fully booked?

Not today, but it's a good idea!
I will add this to my "long" todo list, even if i don't know yet, when i will be able to work on this option. (maybe using the same email, but adding the remaining tickets in admin emails and this way whanging the message to event complete, when no tickets left...)

Is it possible to enter multiple e-mail addresses in the custom e-mail list? What's the proper way of including them then?

Yes!
In the Custom list option, just enter each email separated by a comma (note: when hover your mouse on the label of each option, you will have some help and information )
eg. :(with or without space after comma. The comma is the separator, and spaces are trimmed in code )

Hope this helps ?

Best Regards,
Cyril