So after a little iterating, here's a quick improvement that takes some cues from the progress design without completely rethinking the topics index*. The mobile stuff is still soft-launched so I feel ok pushing this to production so we can review it with the iOS app shortly. Just wanted to see if there were any gut reactions without having touched it.
*It's really easy to fall into the trap of over-thinking this design and get back into a topics stream design rabbit-hole. I actually went down that road the first time before settling on the safe path and just replicating the inbox design. For example, there is some ambiguity about how the attachments shown relate to the excerpt (sometimes) and that the comments count refers to the topicable. What I'm telling myself is that it doesn't matter if you don't get all the intricacies of what's going on here. The key jobs here are: seeing what's new and finding something you saw before. The second job seems well-served by the way the titles, excerpts, and attachments give you a flavor of the conversation even if you can't grasp how much of it you're seeing with each entry. It's like looking through a keyhole, but you really have to go in to get the whole story.