What I am currently planning is to set up a continuous meeting from 12:00 to 19:00 CEST. I will record the whole meeting, then cut it into relevant sections and share those to YouTube.

Any presenters can simply share their screen. Just to be sure, it is probably a good idea to send over your material in case of technical difficulties, but Merri is on top of that.

Regarding the invitations, I wonder what would be best: a freely joinable link, or advance invitations by email? I'd probably go with the former, unless someone things that would spell trouble.

Can't see why it would cause problems, you can always kick off trolls.
Keep in mind the max sim users limit which might limit things a bit, especially if you're using free zoom.

I have paid zoom for my YouTube videos. With free zoom this would be impossible since it kicks everybody out after 40 mins. 
So we will just use a link that is free to join. If we reach a maximum number of attendees I will consider the event a great succes