Re: Ki and Remaining Grounded
From my understanding, ki is not something that can be "used". Ki is always flowing, but when we do things such as having bad posture, not eating right, not sleeping right, not breathing right,etc., we block or diminish the flow. Ki exercises help to increase the "Ki power". So, with proper practice, it should always be there, not some power that you try to summon forth to enhance your technique or give you power.
In those demonstrations where people are picked up, and then force the attackers to drop them, I believe at first that are demonstrating a stiff and unnatural posture making it easy to be picked up. After that, they once again assume a natural posture and then it becomes difficult to hold them up. However, I believe terms such as "unbendable" and "unliftable" are not meant to be taken literally. Ki is part of nature, and so is bound by the laws of physics.
As for being constantly aware, well, in my opinion, if we were, we wouldn't have a need for ushiro techniques. To me, ushiro techniques reflect the unexpected, unpredictable, unpreventable, and unforeseeable parts of life. So, I accept the possibility that someone can sneak up behind me and that once I'm in that situation I'll just have to deal with it.
That was the long answer, so here is the short one; Does one have to be grounded to use Ki energy? No. If you didn't have any Ki whenever you left the ground, you'd be dead. Can you use your Ki energy to get out of a situation? You'd have better chances using the force.