We are not talking about the rules violating type of "metagaming" where you play with people you know outside of the game and use out-of-game interactions to get an in-game edge. Maybe we need a different term for "the metagame" to avoid confusion... You're right: this isn't metagaming because ther...

In face-to-face gunboat games, where orders aren't limited by a user interface, you'll even see people write blatant messages to other players like "Army Serbia support Berlin-Baltic". See also Simon Szykman's 1995 article, "Communication in No-Press Diplomacy"

This case is covered in the Game Rules FAQ ("What happens if I order to build/destroy two units in the same territory?" and "If someone has to destroy a unit, but doesn't enter destroy orders, what unit is destroyed?"). The engine will accept the first disband, and then pick another unit for you.