1) Yes
2) See 1)
3) More to do with the point at which you must have the minimum viz/RVR for continuation of the approach. UK Air Law/JAR-OPS says that you must have the required minimum visibility for the approach by the 'Outer marker or equivalent position' (ie. about 4d)
For approaches with neither Marker or DME the 'cut-off' is 1000'aal.
If the vis drops below min RVR for the approach once beyond the MKR/4d/1000'aal, however, then you can carry on to the DA/MDA and land if you have the required visual references once there.
Also used as a glideslope/range/altitude cross-check but I don't know if that is a primary purpose for it being there.
Anyone any other ideas?
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...proceeding below Decision Height with CAUTION...