Originally, I ruled the bonus/penalty cancelled as well, but after much research on this topic, my final finding was CoC does not provide rules for shooting at an enemy you are engaged in melee combat with simply becaue the rules don't intend for you to be able to shoot someone you are in melee with. If you are engaged in melee, you must use a fighting(x) skill. The rationale seemed to be that if someone is close enough to engage you in melee, they are close enough to disrupt your attempt to shoot them and you have to resort to melee attacks. I can't recall atm what my conclusion for "shooting someone you are not in melee with while you are engaged by someone else" was, but I think I still allowed that.
Disengaging takes an action, but automatically happens (unless you are playing pulp). So assuming you are slower than an enemy but they do not have a dex 50 higher than yours, this allows you to disengage then shoot at the +50 dex bonus before the enemy can act. If you are faster than an enemy, I think you can get locked into a disengage-engaged loop, but hopefully you already shot them on round one?