So a topic-level bookmark can’t overwrite a post-level bookmark, if a specific post is bookmarked the post and latest reading position are the two links within the bookmark. If the topic is bookmarked then it’s the topic root / and the latest reading position.
I bookmarked your earlier reply, so my bookmark link goes to /4 and my topic link goes to /6 because it’s my latest read position.
But… topics and posts are such different concepts if you stop to thinking about it.
If I bookmarked a topic to read later/any day and it has 50 replies, I’ll only be able to bookmark my location while I’m reading if I scroll to the bottom and then clean my bookmarks. So it should be the way around… if I’m bookmarking a post in a topic that was already bookmarked, it should override it.
Your criticism of /bookmarks stems from not understanding the priority for the icon. You aren’t limited to a single bookmark per-topic:
If the topic and subsequent posts have been bookmarked, then the icon on that page goes to the topic
If multiple replies have been bookmarked, then the lowest-numbered reply within the topic to be bookmarked becomes the destination
The bookmark icon at the bottom of the topic being swapped to ‘clear bookmarks’ isn’t entirely helpful, because if you bookmark a topic and then a post within said topic there’s no obvious UI to remove the bookmark from the topic, while leaving the post bookmark intact. For now you need to navigate to the above link and remove it from there.
I asked you if you were expecting /bookmarks to show all bookmarks within a topic as discrete rows, because in that view it only shows the topic title, unlike /my/activity/bookmarks. If /bookmarks were to behave in that way, how would a user differentiate visually between two bookmarks within the same topic without having to check the reply ID it directs to. Here’s a mockup:
How would I be able to tell from title alone which of those takes me to the topic, and which takes me to a post?
My last point related to whether it would be possible to indicate the position of all bookmarks along the topic progress bar. In particularly large topics it would be invaluable to jump between them, otherwise users would be forced to jump back to /my/activity/bookmarks periodically to find the next link.