First, I would suggest that you save all Athens days for the very end of your trip. Ideally, walk out of the airport, get you car and go. Delphi is only two hours away and is a good first stop.
Two days is more than enough for Delphi though I do love the site and village. Two nights is fine though you might wish to take some time to visit Hosios Loukas monastery and/or Galaxidi since the site, temple and museum in total are good for no more than five or six hours, IMO.
Then, drive to Meteora. You can see all of the monasteries if you're staying two nights. Just be aware that on each day a different one is closed. Thus, if you arrive on a Monday and Monastery A is closed on Tuesday, make sure to visit that one as soon as you arrive.
Driving a 9-person vehicle should be no problem. I've done it many times without incident throughout that region.
Next, I see no reason whatsoever to spend time in Igoumenitsa unless it's to take the ferry from there to Corfu. It's a concrete port with little else. Ionnina is an interesting possibility since not many US people go there and is worth considering. The castle is interesting, the archaeological museum is nice and the setting on the lake is nice. If you do stay there I would strongly recommend a day trip to hike either the Vradeto Steps / Vikos overlook with a stop at the great Turkish bridges along the way OR to Vikos Gorge itself and a hike up the Aoos River to the Stomoiu Monastery.
The coastal town of Parga is spectacular, though, and I would choose that over Ionnina. It's a bit touristy but I think for a reason. Great beaches all around, too, with a fun day-trip boat out to Paxos Island. I give my MOST ENTHUSIASTIC recommendation to a beach just south of Parga called Alonaki Beach near the village of Fanari. Thank me later; I'm not kidding.
Then, get your island time at Lefkada, home to absolutely spectacular beaches of all kinds. Plus, it is connected to the mainland by a causeway so any ferry delays or issues are avoided. I like the town of Nikiana on the east shore but the more-touristy Agios Nikitas is very quaint. Too many world-class beaches to name but Porto Katsiki will take your breath away (arrive early) and Kathisma is the best overall beach experience on the island (among many). If you're near Nydri you could rent a boat and cruise to many great secluded beaches and coves to enjoy the perfect waters, too.
Lastly, a long drive will take you to Nafplio but you can break it up with a stop at either Nafpaktos, Ancient Corinth, or Ancient Mycenae (though you could hold that for the next day and combine with Epidarus). Consider having lunch at the cute port town near the theater at Palaia Epidavros.
Then, you are about two hours from Athens to end your trip. Let's see, you have Delphi - Meteora - Parga or Ioannina - Lefkada - Nafplio - Athens. That is six stops so with two nights at each you have two to add (I would add in Lefkada to have a more genuine beach experience, followed by an extra night in Nafplio since it has so much to offer) or you could add another island.
Doing so is delicate, though, since you'll have to work out the ferry schedules (both ways - that's why I love Lefakada on a short itinerary).
Two days is enough for an introduction to Athens. Plus, having spent 12 days driving around Greece you'll find the city rather off-putting, I feel. Of course, the Acropolis and other boffo sites are superb but after being in those supremely atmospheric places the city will be a big turn-off. But it's important to spend your last night in Athens.
