Tentative plan - any comments?

Ugh...just saw that Korcula to Split ferry is not running Oct 13. Is this a workable reversal? Note that Oct 8 is Independence Day.

Oct 1: Fly NYC to Zagreb (stay Zagreb 1 night)
Oct 3: Rent 1st car and Drive 2 hr to Plitvice & hike park (stay Plitvice 1 night)
Oct 4: Hike park. Drive 2.25 hr to Trogir (stay Trogir 1 night )
Oct 5: Drive 40 min to Split and drop 1st car (stay in Split 1 night)
Oct 6: 7:40 am Ferry to Korcula (stay Korcula 2 nights - swim, kayak, bike)
Oct 8: 10 am Ferry to Dubrovnik (stay Dubrovnik 4 nights - swim & kayak, weather permitting)
Oct 12: Rent 2nd car and Drive 2.5 hr to Mostar (stay Mostar 1 night)
Oct 13: Drive 2.5 hr to Split and drop 2nd car (stay Split 1 night)
Oct 14: Fly Split to NYC

You are correct that your first option won't work, as the boats will have switched to their winter timetable. There is a reason for this - the weather is likely to have broken and many places on the islands close at the end of September. Also, much of the accommodation has a 3 night minimum stay.

You have lots of one night stays, which makes for a tedious trip.

Have a look at the Jadrolinija catamaran in addition to the Krilo catamaran. (Neither are ferries.). You could get their late afternoon catamaran to Korcula, rather than having a one night stay in Split to get the early morning Krilo catamaran. Have 3 nights on Korcula. You can take the bus from Korcula to Dubrovnik, giving you more flexibility.

Check prices, but I would envisage that two one way car drops are going to cost more. Check that you can take the car out of the country. Some may not allow this or charge more.

Your driving times look optimistic. If using Google, add at least 30%.

Would another option be that you didn't back track and flew home from Dubrovnik? This would make a more logical trip.

The holiday falls on Sunday thus year. I don't know what that means about closures. I can tell you that in 2015 the holiday fell on a Thursday, and it seemed that many, many people took a 4-day weekend. The streets in Zagreb (normally a very lively city) were deserted. Fortunately, I had already completed my museum agenda (the city has many).

Hi Jennifer, Yes, I keep reading about the three day minimum on the islands but still see plenty of stuff on booking websites and bnb websites that do not have that requirement. Plenty of two-night confirmations available. Not sure where the 3 night minimum is from but it is not borne out by the sites that I've checked. Not sure another day on Korcula is what we're looking for. Our frequent flyer miles program and its partners do not fly out of Dubrovnik so we were trying to stick to Delta, KLM, AirFrance by using an open jaw NY-ZAG-SPU-NY

Deserted streets sound great to me! I live in NYC and am not looking for heavy foot traffic. Seemed like that would be a good island or driving day, though.

I don't think the three day minimum on the islands applies off season, only in the peak summer months.

Ah, a minimum in the high season would make sense. Might also reduce "party culture"

Just found out that the public catamaran goes from Korcula to Split every day! It's only the private boat that changes schedule. Back to my original 1 car plan. Phew! Look at www dot jadrolinija dot hr http://www.jadrolinija.hr/en/schedule/LineSearchResults/Index/2331754/2331792/13102017

I knew there was a daily catamaran - thought you meant it wasn't going to run on a holiday.