Yes the KI system can seem intentionally slow. And you're right, there is a huge meta in the combo system in itself. If you guess/or react right, you can get huge damage or prevent the same from happening to you. The ABC anology is a good way to look at it. Manuals is where you can really break that up for the opponent. I'm still so bad at them.
As far as bouncing off of things every character has a "wall splat" ender. What it does is enable you to end a combo near the corner in which they bounce off the wall. You then typically perform a shadow linker to continue the combo, since otherwise you would drop the combo due to the combo meter, and then finish with a damage ender for some quality damage!
As far as gravity every char also has a launcher ender that enables a juggle of some sort. The gravity seems to vary based on the length and strength, or level, of the combo. Shadow enders also allow a different set of possabilites char dependant. In my short time with the game it does add on a bit of damage, but nothing massive. Most of the damage is done before that point. The bonus being they can do nothing about it once juggled. It is also admitadly swag.