I'll address the consistency first: If the shoulders and hips are square at impact (where they were at setup), your hands would have had to also returned to that exact position at setup. During a swing there are forces that you put on the club as it's rotated around your body. The hands are going to have a hard time holding off that force and apply the proper timing to get the face returned back to it's perfect square position. By allowing the hips to rotate fully prior to impact, this causes the chest and shoulders to also begin to face the target. If your going to have a square face in this position, the trailing hand will need to be in a cupped (bent back) position. This position is as strong as it can be. This position can hold many more times the force of a position that is straight. To prove it, take your palm face up and hold a gallon of milk in you fingers. See how stable your wrist is if you let it bend all the way back verses trying to keep it straight?

As for power: Acceleration through rotation is a good source of power By allowing your hips and shoulders to fully rotate, you have maximized your potential and range of motion for those rotating parts. If you try to have your acceleration timed to meet at that one point (impact) and there are further positions that that rotation can go to, you will have to be extremely good at timing each part. This causes errors, that contribute to losses of power. If you are only worrying about making a full rotation with each part, then there is no "timing" to think about, only just "get there" position.

As for your trying to incorporate one aspect of the swing (just the hips rotating more), can cause you to get out of sequence. This is a critical part of a powerful swing...I use a bullwhip as an example. Thick handle and a thin tip. Throw the handle hard, and the rest takes care of it's self. The legs/hips are the handle, and the hands are the tip of the whip. Everything else in the middle has to go in sequence causing the other to start it's whip effect. This is easy to feel once you get in a relaxed state. Any items that is tense will fight the affect of the preceding action. So shoulders that have tension will not benefit from the action that your torso just put on them. Arms that are rigid and trying to hit the ball hard alone do not get the benefit of the full rotation of the shoulders whipping them around. All this simple bottom to top (big to small) whipping acceleration becomes such a wonderful synchronized motion that is easy to repeat once you relax.

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I'm a golfaholic, no question about that. Counseling wouldn't help me. They'd have to put me in prison, and then I'd talk the warden into building a hole or two and teach him how to play. ~Lee Trevino