Ok, but until proven otherwise, these are rules that you’re passing on to each other without really knowing why. If the only argument is to say “that’s just the way it is, it’s the rule,” then it’s not worth much, sorry. And once again, just because Sydney says something doesn’t mean the same rule applies everywhere. After all, everyone can do whatever they want.
But in some countries, you can have two passports for this reason: one is waiting for a visa while you still need to travel. And on the website of the embassy in Cambodia, they don’t mention this, so I don’t understand.
I don’t see why it would be a problem for you to be in Vietnam or Laos instead of Cambodia. Of course, there might be an interview, but if you can come back within three hours…