Duplicate hooks / idea

fn_qr_add_online_user/fn_qr_add_online_guest_user are actually meant to be duplicates. For most queries, we would simply use one hook at the end (before executing the query) and be done with it. However, in this case, we use a different function call depending upon the dblayer. So, we can't so easily put one hook just before the execution of the query (we're trying to avoid using another switch statement to determine which function to call).
But thank you, I've corrected the other duplicates.

Well, I thought I did a pretty good job of explaining the duplicates above, but I'll try again.
Notice where they're used. They're both within a switch statement, in the two different cases, right before the query is run. Normally, we would put the hook outside of the switch statement and then call $db->query_build on the result.
However, in this case, to get around a race condition in our PostgreSQL code that led to duplicate posts, we use $db->query for PostgreSQL and $db->query_build for MySQL(i)/SQLite. So, we can't just put the hook outside and call a function: we don't know what function to call without another switch statement.
In this case, the duplication is almost irrelevant: if we were to put the hook outside and add an extra switch statement, people would have to deal with the code in the exact same way they would now. It would just look uglier from the perspective of the PunBB core code.

As for the subscription idea, I see nothing wrong with it (and it's not difficult, we already store the previous URL), but I want to see what other people think.

For the same reason we don't have one hook per query per dblayer: they're all the same query, just formed differently to take care of the idiosyncrasies of the different systems. In this case, you simply do an is_array check rather than looking for a particular dblayer.