1. Don't need much cable management though it never hurts to clean it up inside for the best air path possible for your fans.
2. It's both easier and harder to install the H100i than the Evo, the mounting system for the Hyper212 line is something I've never a been a fan of, a lot of better options out there and the mounting for the pump onto the CPU socket is pretty straightforward. Mounting the radiator with fans can get a bit annoying if you're mounting the fans between the rad because you have to balance holding the rad up with the fans while you try to get the screws aligned properly. With 2 people fair easy, solo it can get a bit annoying depending on the person and the size of the case.
3. Chances of a random leak springing up are fairly low to be honest, that being said don't work the tubing too much, it should be fairly obvious what the max you can turn it is. The H100i is a closed loop system, meaning it should never just magically run out of coolant inside unless there is a small leak somewhere. Now that being said the most common reason this cooler doesn't remain usable is that the pumps aren't the greatest out there and they do fail.
If you're not planning on overclocking, I think that temp is great for a stress test run and you only do a bit of gaming and other software. If you routinely have it maxed out (rendering and such) might help your peace of mind to upgrade, but I don't see it being entirely necessary for your situation.