As we spent September there, I’ll make a few suggestions.
I’d concur with others on Zagreb, especially if you enjoy museums. Plenty to choose from and variety unlike the Balkan hot spots. But, considering how much you want to cover in 2 weeks, I’d probably move on. Ljubljana is lovely. Check the food reviews. There are lots of really good restaurants. Eat up, it gets boring later on in Croatia and Bosnia, unless you like grilled meat.
On 6/19, your plan is possible, but you’d be rushing things. Bled Castle is boring, but free if you have a restaurant reservation. The views from there are wonderful. The restaurant was good, pricy and popular. Make a reservation.
Your plan for the 20th is, in my view, unlikely to yield a satisfactory result. The Alps take a lot longer to drive than you’d think, and adding stops at the Russian Chapel, switchbacks for photos, bus traffic, and stopping to eat if you’re continuing on, you’d be lucky to make it as far as Kobarid, let alone all the way to Rovinj. Poor planning required us to push on to end the night in Piran, which is a charming little town. We thought the museum in Kobarid deserved more than the hour we were able to give it and still get to Piran at a reasonable hour. Cutting the Alps would be a shame, but you can’t do everything.
Rovinj is a pleasant little burg well-suited to just strolling and enjoying the slow life. But, you’re planning on spending any time in Rovinj, I think adding Motovun and maybe Pula and Groznjan expecting to get to Plitvice in time to get in the park and enjoy any of it is overly ambitious. Maybe it would be better to bypass Rovinj completely and take a hill-town route to Plitvice. We stayed at Hotel Jerezo. The room, although dated, was very comfortable and had a great view. Food in the restaurant was average. The buffet was decidedly less so. If I were doing it again, I’d stay off the property.
Are you sure you want to keep your car in Split, and then take it to one, or both, islands? I’d dump it on the way into town at the airport, visit Split, take the catamaran to the island(s) and pick up another car for getting to Mostar. We, too, had a tour booked with Maja Benzon. She canceled at the last minute and sent an inferior replacement. Since an island stay hit the chopping block, I can’t comment, although the views of Hvar and Korcula from the catamaran were pretty.
While driving to Mostar from Split, which, according to google maps, is about 2.5 hours, don’t be surprised if it takes at least an hour more. By the time you get back from the islands and drive there, it will be (late) afternoon, not conducive to much sight-seeing, although the herds of day-trippers are gone by around 3 pm, which will help.
Personally, I think Dubrovnik is highly overrated. It has an absolutely crushing number of tourists. Other than the wall and Mt. Srd, we found little of interest.
We enjoyed Montenegro and a day trip with a driver and guide from Miro & Sons, but from Dubrovnik it will be a long whole day between driving and a slowdown at the border crossing and coming back.
I know the temptation is to try to see everything. I’d try to eliminate the one-night stops. It’s stressful, eats up a lot of time, and doesn’t really give you time to enjoy where you are. Overall, I recommend the packing trick – lay out everything you want to take, then cut it in half. There’s a great line from a movie, “Sometimes more isn’t better; sometimes it’s just more.” I agree.