I'd like to know wether the brake quick release lever on the Ultegra 6800 can be kept in any intermediate position between completely closed and completely opened when riding.
It should be fully closed. The lever is only intended to be opened to allow the tire to pass during a wheel change or if you hit a pothole and knock your wheel badly out of true. If your pads are too close to the rim you make an adjustment with the barrel adjuster. Riding with the quick release open compromises the braking power of the caliper. You may not notice while riding along normally but in an emergency stop at speed it absolutely will result in slower braking.