I tested the topic preview along with our “timeline” plugin in my localhost. It is just working. But I can’t understand why you need both plugins same time. While topic preview display OP’s excerpt timeline plugin will display last unread post.
Actually both will be displayed.
No. not last post excerpt. It will display whole last unread post. In a topic which totally having 6 posts if your last unread post is 1 then it will display first post. If it is 6 then it will display whole last post. Same rule will apply for any 'n’th post on topic. For guest it will always display the first post.
Also note if you logged in and you don’t have any unread post in any topics then whole topic list will display as normal without any posts and previews. So in this case you can’t find any differences made by timeline plugin.