Thanks, @awesomerobot. I added the CSS (and now how I know how to do that so another Discourse learning box ticked) to show the reply button but I appreciate the enhancement.
Even without knowing how the software works in detail I had a feeling that the ‘last post in preview’ idea would be complicated and performance impacting. I don’t think it matters in many themes but with this one it’s food for thought. Might have a crack it myself when I get to that level of expertise.