First, reduce the number of speakers as much as you can. The trick to effective webinars is keeping them short and as such, too many speakers is an issue.
If you are looking to do more of a panel discussion, I have sued Zoom and it worked well. But there are plenty of Webinar platforms out there. Like Demio, WebinarNinja, WebinarJam and Clickmeeting. There is a good overview here.
Set it up yourself and do a test with one other. Then rinse and repeat with the rest. It isn’t too hard.
What is more likely to be an issue is the other speakers’ network connections. This is where you have to insist they are on a high speed, connected (not mobile) connection – or not on the Webinar.
It’s really worth doing a rehearsal if you aren’t using people that do this all the time. Record the rehearsal and let them see how they sound. That normally gets people nicely sobered-up and focused on being brief and to the point.
And remember, no less people watch a pre-recorded/automated webinar than do a live one. So, just prerecord and remove the angst. Then do live Q&A if you need to.