I like the relatively low key moderation here,
That's a positive surprise to hear that ;).
A couple of days ago I created a thread dealing with pronunciation. In my mind the topic deserved to have its own little area. Within less than an hour it was merged. The validity of the move could be argued both ways, but that's not the point. To me it's a matter of courtesy, so it was disappointing to first see a useful thread disappear altogether. I stumbled upon what I posted later, but now in another thread.
As you noted, "Merge" doesn't equal "Disappear." There are many posts that we feel should be kept in one thread - especially when they are similar and of the same author. For example, there's Youtube thread by Janusz who presents his videos in one thread - that way it's easier for the reader to track the posts.
Is there a "move" and/or "merge" topics function available to mods?
Surely there is.
Like others before here, I would like to see some info from mods as to why a thread has been locked, or deleted (although I don't agree with thread deletions at all, as there is some educational value in those those too)
In most new "cases" we put a comment or warning right below the post in question (in small font). But it's not possible to explain that in every situation. Generally, as noted above, only threads from the Off-Topic forum may qualify for deletion.