If you wan't to make the escorts join the TF, then they will have to be from a seperate group, and you will have to time the meeting perfectly. Good luck. AFAIK, all of the ships in a group never try to converge, they just follow a leader an try to maintain their orientation relative to the leader as the group heads from waypoint to waypoint.
The aircover shouldn't be much of an issue, I suppose they may try to attack each other, but this will be determined by AI and there is nothing you can do as a mission designer to influence this.
The primary mission objective is impossible as stated. The game does not recognize when any ship except the human controlled sub enters a zone.
You could however, create an objective thet fails if any of your ships are sunk and succeeds when all of the enemy units are sunk.
The mission editor is a bit too primitive to allow for highly creative or intricate maneuvers, I'm afraid, and there are only a limited number of objectives that are recognized.
