Well.. You couldn't put [nocombat] as a conditional unless you do manual IWTs at the start of each fight (or your melee toons will avoid joining combat althogether) and once they've started running, ending the macro isn't going to stop them =D.

The other stuff might help, best bet is to try it.

Outside PvP is probably not worth spamming IWT anyway. I find a few well timed presses of a manual "Melee IWT" key map most effective at avoiding randomly running toons.