Whether you think it’s a horrible idea or not, having a single [Course Forum] thread is the rule that Memrise established and communicated to us. We don’t have to agree with them, but it is their site and their decision to make.
It also happens to be the most logical way to utilize the notification features of the Discourse forum system that we have now. It is the only reliable way for course creators to receive immediate notification when users post problems about a course.
When users post comments about a course in another random thread, or create new threads about each problem they find, the course creator will never be notified. And unless every course creator checks these forums several times per day, they will likely never see those posts, or get the opportunity to fix the problems that users report.
Regarding merging posts, the two latest instances that you were complaining about, Maxine_Downunder’s posts about “conduite/comportement” and “la retraite”, both clearly relate to the same course - “Intermediate French by Coralie”, and both of them should have been merged into an official [Course Forum] thread for that course. Your complaint about merging those posts is totally unjustified.
When these forums were first implemented, some of the moderators were inexperienced, and acted far too hastily to merge, close, and delete threads, and some occasionally still do, but that has improved with time and experience. Being human, they are bound to make mistakes, and when they do, there is nothing wrong with politely bringing it to their attention.
But being disruptive, and flouting the site owner’s rules about where posts belong is not productive, and should not be encouraged.