Last year I had no problem buying walk-up bus tickets in Slovenia and Croatia, but there aren't many buses going across the borders, so it would be smart to try to get those tickets a little earlier if you travel by public transportation. I did not travel directly between Zagreb and Ljubljana, however.
I fear that your Plitvice experience is not going to be great if you drive in from Zagreb on the day of your visit. Try to spend the previous night in the park so you can start walking earmy to beat the crowds. There are public buses from Zagreb thst can get you there and back.
You don't need a private car and driver, or even a rental car, for your itinerary in Croatia and Slovenia, but I can see the advantage for the day-trip to Kotor, assuming a vehicle that accommodates your group is available, with or without a driver. I don't know that a bus tour would be much cheaper for 5 people, but tours from Dubrovnik are available, at least during peak season. When is your trip?
I suggest checking online cruise-docking schedules to see what the situation will be like in Dubrovnik and Kotor at the time of your visit. If both areas are due to be swamped, I'd skip the town of Kotor, which is much better with no monster ship in port. There's no sense spending two days among the cruise crowds. There are other nice places in Montenegro (including the drive around the bay), or you could go to Mostar.
Check the online schedule for the Naive Art Museum to be sure it will be open when you need it to be. One of the problems with a whirlwind trip like yours is that you have no recovery time if something you want to see is unexpectedly unavailable. Personally, I'd drop Ljubljana and Bled from this trip and hit them at a later opportunity. You are not going to get the best out of these destinations because you will be constantly rushing toward the next place.