Have you researched weather stats for February? Zagreb and Ljubljana are likely to be cold.
You need to check all of your travel legs for feasibility on ViaMichelin.com. The usual suggestion is to add a bit of time because VM is too optimistic. And understand that VM doesn't include any stops at all.
Day 1: Zagreb is worth more than a few hours. It has some interesting museums; check online to see whether there's anything that might interest you for an hour or so. The Museum of Naive Art is small and very good. It's in the lovely historic district, which is where you should spend the rest of your time anyway. Unfortunately, you won't have much time before it gets dark. I hope the area is well lighted so you can still enjoy it.
Day 2: I think the most logical route would be Zagreb-Pula-Rovinj-Motovun-Zagreb or the reverse. But ViaMichelin estimates a total driving time of 8 hr. 13 min. That includes no stops, no searches for parking places, no walking from the parking place to the area of town you really want to see, and no time to eat--and you've already used up most of the daylight you'll have in late February. I love Istria, but this is not a good idea at all. Use this day elsewhere. Zagreb?
Day 3: No, not practical. Ljubljana is a 4-hour roundtrip from Zagreb assuming no delay at the border (or weather issues), and Lake Bled is close to a 2-hour roundtrip from Ljubljana. When are you going to actually see those destinations? Ljubljana is very nice, but to drive 4 hours for half a day there--I wouldn't do it. Note: Motovun to Ljubljana is closer than Motovun to Zagreb if there are no border issues (a bit less than 2 hours vs. 3:13, discounting border controls). That's not enough of a time saving to make Days 2 and 3 work, though.
Day 4: Driving time Zagreb-Plitvice-Zadar is about 5 hours, so this could just barely work. Normally I'd insist that a mid- to late-morning arrival at the crowded park would be disastrous, but in February you might be OK. But only make the stop if you have unusually good weather. There's no cover out on those trails. If it has rained recently you'll be walking on slick leaves. There are virtually no handrails. It's not truly dangerous because you're usually only about a foot above water level, but falling in, in February would be extremely unpleasant.
Day 7: VM says about 5-1/2 hours from Split to Mostar and on to Dubrovnik. But there are two border crossings in there. Mostar is very interesting. I'd consider spending the night there so you don't have to kill yourself with another early start and find your way to your Dubrovnik lodgings in the dark.
Day 8 - 9: I don't know about island transportation in February. You may have some luck with Google. If nothing looks practical, you don't really need two days to see Dubrovnik, and one of those last two days could be used earlier in the trip so you have a chance of enjoying Zagreb or Ljubljana. Or Istria if you are unwilling to give it up.
The above itinerary isn't really doable even with a car. It's hopeless via bus and the rare train in the time you have available. Only researching the schedule for each transportation leg will tell you how much of it you'd be able to pull off. Weather permitting, of course. Try BusCroatia.com for buses and the Deutsche Bahn website for trains.
You need the permission of your rental car company (possible extra fee??) to drive into Bosnia-Hercegovina to visit Mostar.
I travel in winter only if forced to do so, and never to Europe. I have no personal experience with winter conditions in Croatia, but I have my doubts about visiting Istria, Plitvice, and the islands in February.
