Flat surface will help with dribbling but you could do a lot of other things that don't include dribbling on an awkward surface. We used to run a series of drills passing the ball between legs, around the head, etc never touching the ground. We'd also incorporate bounces but at the very least you could so some figure 8s between the legs n stuff. More you touch it the better far as I'm concerned.
Is it okay if I practice my handles in my backyard where it has a hard concrete floor is a little slanted? Or will all that practice not translate to a court in a gym since the floor is way different?
As long as you are dribbling the ball, it wil help. In some cases dribbling on those awkward types of surfaces have actually helped players even more, something related to forcing you to concentrate more.
It's not carrying unless your hand comes under the ball so you can learn to guide the ball as you spin. Also you can use a dribble as part of the spin where you basically angle the bounce so its waist high when you're back is to the basket but it ends up at your left hand when youre back around. spin moves are hard to discuss in an internet forum o.o
Cheese as it sounds go look at animations for dribbling in like NBA 2k.