10 Days Eastern Adriatic

• Ljubljana 3 nights (day trip to lake bled)
  I haven’t done it, but I think that sounds perfectly reasonable: arrival day (N), see the town day (N), daytrip day (N). For Bled you could do a train or a bus or join a small group tour (check the resellers like Viator and GetMyGuide for ideas).
• Plitvice 1 night
  With so little time on the holiday, it’s a big decision to tie up a night for this. But you come from Ljub so that impacts the decision and if you want to spend 6 hours or more walking the trails then that does make it the right choice.
  So, a private transfer at 8am. Three hours to Rastoke for an early lunch and sightseeing. Then walk the lakes from 2pm until 8pm. Check into the hotel. The park hotel is like 250€ a night. There are cheaper further away but you will need to check and see if they have transportation to the lakes. Then the transportation to Plitvice is maybe 250€ including the stop in Rastoke (for the high end, check Daytrip.com). So, I am guessing 500+€ for transportation to Plitvice and the room.
  
• Mostar 1 night
  Great overnight trip. The trouble is I don’t know if you will be able to find a small group tour out of Plitvice, or even a bus? Maybe? If not, you will have to hire a private transfer. You need to visit Blagaj Tekkesi monastery on the way to Mostar and you need to stop at the Kravica Waterfalls and go swimming on the way to Dubrovnik. The two stops add a lot of depth to the trip and get you into different environments. There are a couple of really interesting villages and roadside vendors along the way as well. My guess is about 400€ for private transfers on this leg, but for my money the extra stops going up and coming back more than make the cost reasonable. If you were to go to Split for a night or two, then Mostar and then Dubrovnik you could maybe do this with a bus or a small tour group and some of those make the same stops.
  
• Dubrovnik 4 nights
  Certainly and it gives you time for a day trip to an island … maybe two.
  Okay, you apparently can add time to the trip. My only comment is that while you hit many of the big attractions you are missing a number of the fun smaller stops. So rather than adding a day in Trieste, I would spend it on the trip you described.
  As for renting a car. Sure, a lot of people love that idea. In this trip you will have drop off fees as you really can’t do circles or you lose too much time. Just picking up and dropping off the car might cost you 4 hours of sightseeing.
  

Here is my favorite trip planner for the Balkans. She can get you quotes on transportation maybe better than what I guessed: Dijana Krkotic, Doclea Travel, +382 69 277 749 (WhatsApp, Phone, Text), [email protected], www.docleatravel.com AND here is my favorite guide in Dubrovnik Nera Pavlovic, Independent Tour Guide, +385 98 183 4365 (WhatsApp, Phone, Text), [email protected], https://www.linkedin.com/in/nera-pavlovic-b2186873/ And if you search the forum you will find some other recommendations.

I'd be shocked to find any sort of public transportation between Plitvice and Mostar, though a private transfer can be arranged, I'm sure (at a significant cost). Most people visit Mostar from Split or Dubrovnik.

Ljubljana is connected to Zagreb (an attractive destination) by bus and train. There is bus service between Zagreb and Plitvice, and there is bus service between Plitvice and Split (though I haven't taken one of those buses). I believe there is also public-bus service between Split and Mostar as well as between Mostar and Dubrovnik, but I haven't verified the current situation.

I don't rent cars in Europe, but it seems to me the most efficient way to make this trip would be to pick up a car in Zagreb--or another Croatian city convenient to Ljubljana--and drop it off in Dubrovnik. I'm assuming the drop-off fee wouldn't be terrible and taking the car into Bosnia and Herzegovina would not be prohibited. I could be wrong on either or both counts.

Acraven has identified the problem points—you are bypassing the transit hubs of Zagreb and Split, which would make your trip easy by public transport. So, if you can add a couple days, you could make it work. Otherwise, you’ll have to make use of a car, whether you or a driver. I never drove in Croatia because my itinerary did not require it, but I’d have no qualms about doing so.
As far as a north v south, I don’t think the temps will vary that much within that time frame, so I’d choose based on flight convenience.