I was going to tweet a reply, but it's too long for that medium so I opened an account to do it here instead. :)
On the subject of key commands, I'd be tempted to abstract the actual command from the action triggered and then use either a config file or an assignment block to map the keys to actions. That makes it fairly irrelevant which keys you choose to use during development, because you're bound to change your mind, and it's too early to get hung up on relatively trivial choices.
As to directions, this depends somewhat on whether combat is initiated between (party) units on adjacent tiles or by units moving onto the same tile. If it's the former, then obviously fleeing in the direction of an enemy would fail - and it is possible you may find yourself surrounded- which won't end well. Simply, you cannot flee onto an enemy tile or one adjacent to an enemy. If it's the latter, then you'd probably need to make sure you can't flee "through" the enemy, so you might want to restrict the direction of flight to that from which you approached battle, and perhaps the directions adjacent to it if diagonal movement is allowed. So if you attacked from the west, you could flee SW, W or NW. Conversely, if you were attacked (enemy moves onto your tile on the AI's turn), then your direction of flight would be opposite to that which they attacked from.
Since you are fleeing, I'd also be tempted to introduce some randomness in the actual direction you flee in (i.e. put the choice to the 'dice' rather than the player's control), while still applying the restrictions above. That means you may flee straight into another enemy, which would trigger yet another battle. You could even link that to the party stats, so that the party may stand a better chance of making a wise decision when fleeing as they get more experienced.
...but you could add that later. Initially, you don't even need to worry about which directions you can flee in, just have a routine for exiting a battle early (without victory) and add parameters and restrictions to it as you add more features to your combat and/or stats.