I don't think the TG's themselves should have the ability to spawn DD's. The ports on the other hand, or any base that supports a TG, should. It would mean that a new spawn option would probably be needed so you can spawn out at the TG. You would have your standard spawn thingy, but a spawn button that says, "Spawn at TG" or, "TG Spawn". So you would have to options of spawning at the port, spawning at the PT spawns, or spawning out at the TG. As for "shutting it down", maybe a dry dock-ish type structure for the Ship "hanger"? This dry dock will be supporting all spawn able ships, which would include the PT's. 
Too complex. Easy and simple solution:
At the moment we have say 4 Carrier task groups. I suggest we make that something like 16 task groups. Four of which can continue to be carrier groups as current (each belonging to its own port). These can remain close to the target area for air support, whilst not being literally at the target.
The remaining 12 are task groups with no carrier,
which cannot spawn planes or amphibious assault vehicles (maybe PTs only for increased defense). They are basically just a attacking fleet with maybe two cruisers and four to six frigates. They too belong to the same four ports as the CVs. Three to each one.
Would increase the defense challenge while at the same time not making it too easy on the offensive side.
Existing player controls would apply for steering routing and rank command authority. Like normal task groups, once the entire fleet is sunk, it re-spawns at the port it belongs to.
Easy-Peasy!
