You'd have what appears to be a 128GB image (or some large fraction of that), which in reality is largely holes (typically: repeated blocks of ASCII 00 bytes).
Of course, you'd need to avoid actually trying to fill that filesystem.
You're better off traveling with a wiped phone, and restoring from backup after you've crossed.
An activity-generator might help address that.
Firing off as part of a duress key entry, and removing itself (from the decoy partition) as its work is done, would suffice.
ADB / forensic tools would be ineffective if USB access is denied (as discussed elsewhere in this thread).
Well no, because if you gave the pin, you'd expect the phone to work normally, including enabling adb. If you gave the pin but adb doesn't work that would be massively suspicious. Same if adb worked but logs were scrubbed. Otherwise you're back at "border guards found out you gave a duress pin, now you're being prosecuted for tampering with evidence".