This doesn’t solve your problem, but you can make unanswered topics more visible with the Unanswered Filter theme component. It adds an Unanswered button to your site’s navigation bar.
I’m not sure about the issue of being able to comment without answering. I do that all the time by creating a normal Discourse post. Usually it’s to ask for clarification of the question, or to try to get some details from the user that could help someone more informed than I am to answer the question. I don’t feel that this requires any special UI.
You can distinguish the accepted answer from other posts in a topic by encouraging questioners to mark the best answer to their question as its solution. To do this, make sure you have enabled the Solved plugin, and that it is either enabled on all categories on your site, or that it’s enabled in the category people are asking questions on.
A feature to auto bump unanswered topics sounds like a good idea to me.
It’s important to remember that Discourse is designed to be a platform for discussions. It is not question and answer software. Question and answer systems are great, but they do put a lot of restrictions on discussion.