Jasper,
There is too much confusion with the 50% rule. Your last reply will confirm that. I have always been in favor of separating the rules for all-in's because of this. The only time a short bet can legally be made is when a player is all-in. All other situations in every form of poker requires at least a raise the size of the bet, or full raise the player is facing.
The other rule (half the bet) is for a player that puts the incorrect amout into the pot, whether it is a short bet or possible raise, based on the amount wagered. IMO, This should apply in all games, but it does not.