I urge you to reconsider Plitvice. You don't have to hike much, you just walk on boardwalks to see the waterfalls. It is really quite a wonder, but if it will be out of your way I understand skipping it. I just hated for you to miss it due to concerns it involved much hiking.
Honestly, at this pace--in the Croatia portion--I would not rent a car. You won't have much time to stop anywhere, you'll spend valuable time looking for parking, and you'll have to get accommodation with parking when staying in less car friendly old towns will be preferable.
All of these routes are well served by bus, and then you won't have to take the car on the ferry--something you would have to get permission from the rental agency to do.
Given the time constraints, I think I would leave out Rovinj. You can get from Zagreb to Ljubljana by train.
Good luck!.

Have you looked at the drop charge in Slovenia--may be big?

Good point from bob, and another reason to head to Zagreb to drop the car and switch to train for Ljubljana. Plus, you should plug some of the drive times into google maps or viamichelin. If you were thinking of taking the ferry Split-Rijeka, note it takes forever and a day.

I investigated picking up a car in Croatia and dropping it in Slovenia, and there was a substantial drop-off charge.

I reluctantly gave up Slovenia for my trip. But you could drop the car in Porec (north of Rovinj) and take a bus to Ljubljana.

We loved Plitvice (a World Heritage Site) and agree you could manage to see some of it without hiking. I wouldn't detour all the way inland to Zagreb however. We drove from Plitvice to Rovinj in an afternoon including a lengthy (inadvertent) detour through Rijeka.

How interested are you in Split? We spent a morning touring Diocletian's Palace, had lunch, and drove to Plitvice in the afternoon. Stayed the night at the Hotel Jezero in the park, toured the park in the morning, had lunch at a large hunting lodge-style restaurant, and drove to Rovinj, arriving about 5:00.

So if you curtail Split, you'd gain a night for Plitvice and another night that I'm sure you can use along the way. Maybe more time in Rovinj exploring the Istrian peninsula. Or a night in Trogir.

I agree with much of the advice you've been given:

Do consider using public transportation instead of renting a car - or keep your rentals to the minimum for your purposes.

Do consider the Plitvice Lakes even if you are not hikers. It is spectacular, and parts can be seen without a lot of walking. If you are sure it won't suit your needs, you might consider a visit to the Krka National Park outside of Sibenik instead. The nearest town is Skradin; from there you can take a boat along the river to the entrance to the park. There is a nice walk that takes you by several lovely waterfalls, but the main waterfall - Skradinski Buk - can be seen from just inside the entrance, and it is a stunner!

There are car ferries between Split and Hvar, but I'm not sure how often they sail at that time of year. Even if you do rent a car, if you are committed to visiting Hvar, you might want to explore options for leaving the car somewhere so you have more ferry options. I have no idea what those options might be - just throwing the idea out there.

I think your time allotments are generally reasonable, but of course that depends on what you want to see and do. For the record, I'm one of the people who loved Split and found more than enough to keep me busy there, including several very interesting museums. Ljubljana is absolutely lovely, but you could probably cut a night there.

Hope this helps!