Since support for the webhook endpoints is apparently not going to be provided through this forum, I decided to accomplish my objective through a different method.
I requested enabling meeting archiving from support, and eventually got this response:
I then set up another legacy app for Meetings SDK. I used the credentials from that app to process the meeting.ended webhook endpoint that I am able to receive. I just take the meeting uuid from the payload from that endpoint and use it to get the chat messages from this API endpoint.
I still have not been able to get files uploaded to an in-meeting chat, but I have another devforum post here that I hope will be answered to resolve that issue.