Travel over the holidays

Check wunderground.com (History > Monthly) for day-by-day historical weather stats in December 2017, 2016, 2015, etc. However, you don't have enough time to adequately cover the places you are proposing to see. Your arrival day is pretty useless for anything more than wandering around aimlessly, trying to stay awake. Mostar is at least a full day. The Bay of Kotor is a full day. Getting to islands may be trickier in December/January because transportation laid on for tourists will be much less frequent. My guess is that you'd need two nights on each island just because of the schedules (though I admit to never having traveled to Europe in the winter).

Getting to Istria is a bit time-consuming, and I wouldn't go there for less than three nights. Then you have to get to either Ljubljana or Zagreb, both very attractive places worth more than a day.

I was in Croatia last January and had a wonderful time. Many restaurants are closed during this time of year, but there were not the throngs of tourists. We could go to any restaurant that was open without reservations.

Some of the tourist sites were without any other tourists. Krka NP only had a couple of other people: I literally only saw two other people. The fortifications at Ston on the Peljasic peninsula had no one walking the ramparts except my wife and I.

Weather was mid 50’svwith some days in the low 60’s. I think we lucked out with only a few hours of rain on our day of arrival.
Do take notice that daylight is at a premium. I think that sunset was between 5:30 and 6. I had hoped to go to Montenegro for the day but opted to spend more time in Dubrovnik. IMHO I think that your list of cities/towns is a bit ambitious. There may be a bit more travel than you would like.

We spent time in Split, Ston and Dubrovnik. Split has some great sites and has the possibility of few close Day trips.
Peljasic peninsula has some great wineries, the small but charming town of Ston, and close to I think Hvar.

Just make sure that you have clothes that you can layer and be prepared with a n umbrella. PM me if you want more info.

"Proceeding up the Istrian peninsula" takes a LOT of time. There are no trains. Ferries at that time of year are few and far between, and make multiple stops (if they are running, given the weather). Buses are slow (mine took 4 hours between Split and Dubrovnik.) With only some 8 days or so on the ground, you may really want to focus on southern Croatia, i.e. Dubrovnik (day trips to Mostar, Kotor, Korcula) and Split. It will take you much of a full day to get to Ljubljana from Dubrovnik or Split, and you will need to fly to make it in one day. Even getting to Zagreb would take much of a day. There may be direct flights to those cities on Vueling, Air Croatia, or Adria Airways. Check Rome2Rio.com for your travel options between cities. I booked from my home city to Split on Delta this past June, and they connected me to Split on a partner airlines, Croatia Air, so I traveled on one ticket all the way to Split.

I think that doing Dubrovnik and Split in your short 8 day trip might be the best plan. You could land in Dubrovnik, spend 3-4 days there, and then go to Split. You might consider the bus (not sure if they are running) or a private driver. You could rent a car, but it's a hassle for a single drive. Day trips from Dubrovnik could be done on a bus or car. Dubrovnik is easily walkable itself, so no car is needed there and parking is not easy.

Get a map, and plot out locations. You still have 3 months. Google maps suggests that the drive from D to S is 3-6 hours. If you drive, you could go to Mostar. I don't agree that Mostar is a full day - you can do an afternoon. Hiring a private driver would not be cheap, but would be simpler than renting a car for a one-day deal. When we were in Beograd Serbia, we simply asked at the TI if there were private drivers, and they hooked us up with a good guy who was not hugely expensive, and made our trip much better. Plus private drivers can stop for pictures, find a good place for lunch, etc. You drink, he drives. It's all good.

I'm sure there will be buses running between Split and Dubrovnik, but holiday schedules may be different from regular weekday schedules.

I think the BusCroatia website is useful, but I've found online bus schedules generally somewhat less reliable than online train schedules, and that is definitely true in the Balkans. I always urge travelers to check outbound schedules at the bus station as soon as they arrive in town.

Bus Croatia indicates at least nine trips a day, with extras planned for Dec. 31 and Jan. 1.

On a couple of occasions during my last trip, I stopped in at the city tourist office to ask about bus service farther into my trip (not departing from the town where I was currently staying), and the staffer immediately picked up the telephone and called the departure bus station rather than using the internet.

Also, for your Mostar day, if you don't rent a car, I suggest you contact Ermin Elezovic. His contact number is in Rick's Croatia/Slovenia book (in the Mostar chapter). His email address is: elezovicermin@gmail.com. Ermin has a very large, comfortable car, and drives around the area (including into Croatia). Ermin's wife (forgot her name, but I met her in Mostar - wonderful!!) does guided day trips in Mostar and the area - she is also described and recommended in RS's Croatia book. They also have a couple of other drivers in their business who might be the ones to pick you up in Dubrovnik or Split and get you to Mostar.) Between Ermin and his wife, if either is available, you might find that they can help you arrange a fabulous day trip to Mostar. I am pasting in my description of my day with Ermin in June, 2018:
This was the most beautiful, insightful and enjoyable day of my trip. I pre-arranged with Ermin Elezovic (in the RS book Croatia/Slovenia), p. 429) to pick me up at Panzion Cardak in Mostar and eventually deposit me at the Old Town Hotel in Sarajevo. Ermin is warm, witty and wise. He was dependable, helpful, a great driver, and loves his country passionately yet with insight and honesty. Immediately in the morning we went to the historic gorgeous Sufi site at Blagaj, where the old Sufi Dervish house is built right beside/over a river and beside the cave that is the source of the river. Ermin got us there before any other tour groups or individuals, so we wandered through the house alone, feeling the strength of the ancient stones, the water, the soaring swifts overhead, and the cliffs hanging high above us. He knew about each room, its history and use, and clearly loves and reveres the place. Next we visited the historic old village of Pocitelj, where I explored up high stone stepped streets, and then rejoined Ermin to relax at the shady table of an outdoor cafe run by a friend of his, enjoying the views and the cool drinks. Finally, we had an incredible lunch at a country restaurant down a quiet lane, alongside a small river, that had pools full of fresh trout, and a stone mill where they grind their own polenta. Wowee! For under $10 US I had fabulous grilled fresh trout (those guys were probably swimming earlier that morning), polenta with herbs and butter, salad and bread. Oh my. And we sat in the shade beside a pool, watching other trout swim unaware of their eventual grilly fate. Finally, Ermin drove me into Sarajevo and made sure that I was within a few steps of my hotel. This private tour was my "splurge" of the trip and worth every penny (Bosnian mark).