1. One-word answer: temperature. The smaller the gravitational field gradient the less likely a virtual particle pair (not restricted to photons by the way) is to be split into real particles and the lower the energy carried by each. As photons have zero rest mass there is no lower limit to their energy, which corresponds to no lower limit to the temperature of the BH. All sufficiently massive black holes have a very low temperature. A solar mass BH, for example, has a Hawking temperature much much lower than that of the cosmic background and is absorbing those photons, thus slowly gaining more mass from them than it is losing from Hawking radiation. The hypothetical observer would see the CBR as being much much more intense than the Hawking radiation which would be swamped completely by the background.
2. There is no good theory of quantum gravity as yet. The naive calculation is almost certainly wrong. Further, Hawking radiation has yet to be observed because all known black holes are far too massive to emit significant levels of radiation.