@ludovic, I’d suggest leaving a comment in a thread when you close and merge it into another one explaining why those actions are being taken, since users who see that the thread was closed may not understand why it was closed. This is a good practice in general, but especially in cases in which threads are merged into existing threads in “All Around Qubes,” since many users (especially newer ones) don’t have access to that section, and new users are the ones who are most likely to be unfamiliar with the forum’s rules.
The source topic contained 4 posts, I moved the 4 posts to the target topic, and added a comment in the target topic.
I try to understand the merge topic process with the @adw 's advice.
I should :
In the source topic, add a comment with an explanation for the merge
Close the source topic (topic actions / Close Topic)
Move all-except-the-first-and-my-last posts to the target topic :
The reason for my comment is because I first saw the thread you closed, and it wasn’t immediately obvious to me why you had closed it. (From experience, I figured it was probably either off-topic or a duplicate.) Then, later I saw the existing thread in All Around Qubes that you had merged the closed thread into, along with your explanation about it being a duplicate. But it occurred to me that:
Some users might see only the closed thread and not see the target thread.
Some users don’t have access to All Around Qubes. (In particular, the person who started the thread might not have access, in which case they will be very confused as to what happened to their thread.)
Some users don’t have the experience to infer that it’s probably off-topic or a duplicate, and they might instead assume the worst (“omg censorship!” or whatever).
On the positive side, the closed thread auto-deletes after seven days, so the problem will solve itself. On the negative side, seven days is long enough for negative effects to occur from the above.
So, yes, the process you describe, or something like it, seems appropriate to me. However, there might also be other possible solutions. For example, this problem really only arises because a stub of the source thread is still there for people to see (and get confused by), so completely merging one thread into another (and not leaving just the OP behind with “closed” on it) might solve this problem in all cases except the one where not everyone has access to the target thread (because it’s in All Around Qubes), but perhaps Discourse doesn’t allow for such an action.
I think this might be discourse “tryin’ to be smart”. If one merges all the posts, then it thinks that that conversation should be closed.
Generally I think it’s fine without giving an explanation, I’d say. Here the case is particular because it was moved to All around Qubes, which is not accessible for all. So users who can’t join it are not confused because see a 404 when clicking on the link.
Generally my approach to moving stuff to All around Qubes is to first check if the speakers all can access it. If so, just change the topic to all around Qubes. But that’s not the same as merging, but I’d argue, the process could be similar. If I see it going way off-topic (from Qubes) and the speakers are newcomers that won’t have access to All around Qubes, I don’t want to just take the topic from under their feet. So I don’t mention All around Qubes and what I do here is say to users that the forum should be kept to Qubes stuff and that the thread is probably going to be closed soon because of that.
Then I go to the and set a timer to autoclose the topic in X time. If you don’t have that, you can also use the boomarks .
But you don’t have to do exactly as I do. We learn from each-other and we nudge each-other into better moderating as @adw did it. Setting a moderation culture rather than strict policies is the right way to go, I think.
Lastly, @ludovic, thanks for playing around with the Leader tools
and set a timer to autoclose the topic in X time. If you don’t have that, you can also use the boomarks 
.