I've been to all of these places (except Porec) on different trips.
If money is no object, rent the car in Salzburg and drop it in Dubrovnik. But you'll surely pay a huge one-way multi-country drop fee.
A more practical approach is to start and end the trip in Croatia and detour into Slovenia...or start in Slovenia, rent one car there, take the train to Croatia and rent the second car there, so you'll avoid any multi-country drop fees.
I started one trip in Salzburg and made my way to Ljubljana by train via Graz and Maribor, because I wanted to check out eastern Slovenia. But if you don't want to do that or don't have time, you can get from Salzburg to Ljubljana in under five hours by train. There may be a direct bus (I prefer trains). Flying may even be an option (say to Zagreb to rent the car, but renting two cars may not be a big deal, right?
Ljubljana is one of those places where having a car is a bit of a hindrance since there are a lot of pedestrian-only areas. It's a small city and easy to navigate on foot. One option might be to base your entire time in Ljubljana but do day trips to say Bled or the Karst. You can even rent a car just for the day. You don't need a car to get to Bled from Ljubljana - there is frequent, easy bus service. But having a car is nice, since you can take scenic detours and stop in nice towns like Skofja Loka on the way.
Or start in Ljubljana and pick up your car as you leave town.
On one trip, I took the train from Ljubljana to Rijeka, Croatia, and rented my car there and drove immediately to Rovinj (about an hour from Rijeka). There's a bus directly to Rovinj...but I prefer trains.
If you want to keep the car all the way from Rovinj to Dubrovnik, then you can take the car ferry to Hvar but only to Stari Grad. You could drive from there to Hvar Town (only the catamarans go there from Split). It's up to you whether you want a car on Hvar to explore while you are there vs. parking the car in Split vs. dropping it there and renting a third car (seems a bit much). It sounds like you'd prefer to have a car to day trip to Mostar and Kotor, but you could just take a day tour to each one or even hire a driver (never done it). I guess public transit is possible - haven't tried that either, not sure how flexible the times would be, but I guess it's worth looking into.
Staying in or adjacent to Dubrovnik's old town is desirable, but it's expensive to park a car there. One option if you have a car is to stay in the Lapad area - practical but not very charming - and take the local bus into the old town. Lapad is a pleasant area with restaurants and beaches too if you want some beach time there, but it is really lovely be able to walk in Dubrovnik's old town at night or early morning before the place fills up with mobs of tour groups. I have stayed in Lapad. Some people base in the town of Cavtat (haven't visited) and say it's lovely there too but bus into Dubrovnik.
