Despite "Member" user group doesn't have permission to edit messages in conversations, they still can.
I've tested it by making a test account "GlossyBean" which has the default permission and confirmed it can edit other participants' messages freely.
Damn... this sounds scary, but I have to undergo the same process before my forum gets too big. Its a niche forum founded in 2016 with just over 60,000 posts and about to undergo massive transformation and hopefully rapid growth. So... here goes