SSD reporting incorrect amount of free space, not due to local snapshots
Ok, so after posting this question I had some major breakthroughs, most of which are posted in the update to my question. By then remembering that du still reports in units of 1024 whilst OS X now uses 1000, the final 7 GB was accounted for (well, there's still a potential 500 MB missing judging by Disk Utility, but that's probably just an artefact of du counting in blocks). Now the only question left is why System Information is reporting a substantially lower value for Backups than it should be, and whether or not it will remember to eventually clean up the other 18 GB of local snapshots.