I think your island list can work out well, and the Santorini part can be brief, just for the caldera view. However, because of ferry routes, I would suggest - if possible - that you reverse the first 2 islands. I'm not sure Sept. has a ferry straight from Naxos to Crete & if so it might be smallish passenger-only vessel, rocky in rough seas, frequent in Sept. Possible alternative:
• On arrival Fly to Naxos for 4 nights - then take the 13:00 (1 pm) Blue Star to Santorini - after 2 nights ferry 5:30pm to Crete & etc.
The one challenge is: Unlike flights to Santorini (many per day even in Sept), Flights to Naxos are limited, only 2 per day, one is usually mid-morning the 2nd early afternoon (?). Therefore, you'd have to plan US -Greece flight to be able to catch one of these. This might require flying from Boise to a N. American airport with nonstops to Greece -- Montreal (Air Canada), NYC (delta), Philly(AA) or Newark (a new one UAE). These all arrive between 9 - 11 am... and all Europe connections don't arrive until afternoon... plus, N.America - ATH always arrive on time, whereas with Euro connections, delays occur. These flights are more costly than Euro-connections, but if yr budget can manage it, it would shorten your lengthy journey and simplify your itinerary sequence.
Other plusses for this:
• the earlier you get to Naxos, the livelier it will be (the other 2 have longer 'seasons').
• You'll have your wedding at first stop, then move on.
• in early Sept Naxos hill villages have wine festivals, something fun to take part in.
• Finally, that Blue Star sail into Santorini harbor gives the Famous "reveal" of clifftops ( u can leave parents on deck to gaze, while you 2 hustle down to main deck to the luggage racks).
And there's no need to rent $$$$ rooms in Oia to get a caldera view -- FIrostephani hotels are Also on the rim with FAB views, half the price, AND easy walk to Fira town AND nearby bus stop to visit Oia without parking woes.
Other thoughts:
• bringing 4 parents is great! but it does complicate car rental; vehicles for 6 are VERY rare on the isles, you're probably better off renting 2 anyway, so you'll have flexibility; you already know about the drill on Intl. Driving Permits etc etc.
• Hope u are considering a "symbolic marriage ceremony" rather than a legal civil marriage in Greece -- the latter can be VERy complex and time-consuming to arrange properly.... I think the best & most realistic advice is on this website, run by an American who moved to greece & lives there - http://livingingreece.gr/2007/05/17/getting-married-in-greece-2/ ... the "symbolic" route will give you the memories, without the Greek legalities that can be both costly & frustrating.
• Chania Old Town is the perfect place to stay -- but it can be hard to find one place for 6 people. I stumbled on this perfectly located townhouse apt for 4 -- https://www.tripadvisor.com/VacationRentalReview-g189415-d4520721-or6-House_66_Theotokopoulou_66-Chania_Town_Chania_Prefecture_Crete.html which is right in the Old Section. It might be just the thing for the 4 parents, while the couple has a hotel room around the corner.
