c is 299 792 458 m/s without uncertainty? doesn't work the Heisenberg uncertainty principle for fotons?
The distances typically involved in measuring c are sufficently big that quantum fluctuations are ignored. Also, the length of 1m is defined by the distance light travels in a certain amount of time. Because of this definition, we're effectively defining light's speed, not measuring it, so it is exact.
we define 1m, but we must still measure in how much time light crosses 1m; we measured c but it can be less or more, right?
No, the meter is defined as the distance light travels in a specified time. http://www.mel.nist.gov/div821/museum/timeline.htm