Quick question - is there no way to actually deactivate the "Reply" button on a closed thread? I've lost track of the number of times I've clicked on a post, read the post, read the other responses and then hit "Reply", got all the way to the end of my response and then realised that the thread has been closed.
I realise that you have the little padlock symbol in the left-hand column but I really never look there (and on my phone the symbols are so small, they're almost invisible).
I think that what you want is possible (I am not sure if the reply system has such a feature, but it can be managed). But remember, if the thread is closed while you are on the page (writing you reply), when you hit the reply button you will get an error (that you don't have enough permissions to write in that topic). And you can't do anything about this (in fact, I think it would be possible, but it would be a difficult task).
Not a teacher.
Not a native speaker.
Just a language lover who is majoring in English.