Deniable encryption has gotten way more advanced than this, though - the state-of-the-art is to have a bunch of independently unlockable data stores along with a mechanism to add chaff data that doesn't decrypt with any key. That's how bunnie's Betrusted project works, and it's the same idea as PhonebookFS from yesterdecade.

The main problem with any deniable encryption system is that while your adversary might not be able to prove if you gave them the decoy or real data, they can at least force you to wipe anything you fail to decrypt. In your partitioning scheme, that would mean wiping any partition that doesn't decrypt with the set of PINs you gave them. In the more advanced granular scheme that Betrusted devices use, that would mean border control unlocking all the basis keys you dared to give them, and then them running the storage reclaim tool that wipes all other keys.

In either case, it would probably be easier (and less suspicious!) to pre-wipe your device and then redownload a backup after you pass through border control... assuming you can get access to an untampered Internet connection after the fact, AND assuming your backup is actually complete. Like, I'm pretty sure most apps exclude their login tokens from backup, because every time I do wind up restoring a backup, I have to log into everything again, which makes me wonder what the point of the backup even is?