Split to Krka

You can find the hours here: https://www.np-krka.hr/stranice/working-hours/144/en.html

I don't know the answers to your other questions, since we went to Plitvice and spent the night at an onsite hotel (meaning our admission was included), using Octopus Transfers to get there from Zadar and to get from Plitvice to Zagreb afterward. If it is convenient to rent a car, then that seems like a decent option for you.

Krka is nice, at least what I saw of it. I only saw the magnificent Skradinski Buk cascading waterfall, which dwarfs any one waterfall at Plitvice. I parked my car (didn't start in Split) and took the boat up the river to the falls, then walked all around it, then took the boat back. There is more to the park than that e.g. some Roman ruins - but the attractions are more spread out, and I didn't have time to see more than the falls.

Personally, if you are going to rent the car, I would just push on for Plitvice, which I think is a far superior park. It's an amazing an experience with two huge lakes and dozens of little waterfalls. However, it is going to get very crowded. You might get the car the night before so you can leave very early and try to enter the park before it is mobbed. It would be a long day of driving but it is a pretty easy drive.

If you want to go just to Krka, unless you plan to see more than Skradinski Buk, you might not really need a car - just find transport. You won't need a whole lot of time there. If you do get a car and want to see a really pretty town after you are done in the park, consider stopping in the town of Šibenik near the park - lovely town.