Personally--I wouldn't worry too much about grabbing "second posts" because it'll make the thread a little confusing to read time-wise and it won't bump your thread when you edit it!
Anyway this is such a thorough design--I really like how much detail you went into for this. I think it might be good to think of ways to make this scope-able--either up or down. Since you've outlined a lot of features and ideas you have a lot to pick from, and I think it'll be pretty straightforward to pick a handful of key points from each design element to focus on and develop. This is really thoughtful overall--I'd love to see more!