JAR–OPS 1.405 Commencement and
continuation of approach
(a) The commander or the pilot to whom
conduct of the flight has been delegated may
commence an instrument approach regardless of the
reported RVR/Visibility but the approach shall not
be continued beyond the outer marker, or equivalent
position, if the reported RVR/visibility is less than
the applicable minima. (See IEM OPS 1.405(a).)
(b) Where RVR is not available, RVR values
may be derived by converting the reported visibility
in accordance with Appendix 1 to JAR-OPS 1.430,
sub-paragraph (h).
The above is the JAR chapter and verse.
If you have the runway environment in sight then you can continue because the conditions for continuing below the decision altitude are already fulfilled.
I don't think so. Even if you can see the runway from 100 NM away, the test is still applied at the OM. It's no good declaring a visual approach either, because there is an 800m RVR AOM for that too.
It is not unknown for the RVR sensors to get contaminated, svere frost for example.
And how will a pilot on approach know this, even if correct (it's unknown in my experience)? Regardless, in the UK the RVR is legally whatever the aerodrome says it is.