Would it be possible to add code by each entry that wasn't just an arrow (like in the inbox) but was somehow explained to be "collapse this entry" – that would reload the page with the entry collapsed?

I really don't know any of the technical details, and I don't know how Javascript works, so I don't know if it's any different or easier for slower connections to reload the page instead of loading the entire page with Javascript from the start. And it could be a problem if your reading page had new entries on it between when it was originally loaded and when it was refreshed.

Another question: with the Javascript version described as above, would it be easier and less intensive (in terms of load and coding effort both) to remember the setting while the session is active and forget it when that window or tab is closed?