Trying to make it simple:
Planning:
Ceiling is required.
Vis/RVR is required. In most cases you only have forecast vis. unless dispatching for a short sector based on METAR which include RVR. METARs are valid for 2 hours, provided there's a NOSIG at the end.
In flight, in order to continue past the OM or 1000ft if no OM:
Only Vis/RVR is required.
Ceiling is not a factor unless "Ceiling required" is indicated on the chart.
If RVR is reported, it must be used. Controlling is TDZ RVR, unless it is u/s, in which case it may be substituted by MID RVR.
If RVR is not available, you can use the convert the Vis to RVR, but only if the reported visibility is 800m or more.
If reported visibility is less than 800m, you must have RVR reported.
After passing the OM / 1000 ft, you can continue down to minimums, and if you see the required visual elements you can continue to land.
Examples:
1. Required RVR/CMV 550m:
RVR reported as 550/175/175 (TDZ/MID/ROL) or better, you can start the approach.
No RVR, vis below 800m: below minima, you cannot pass the OM.
Vis is 800m or more: you can start the approach.
2. Required RVR/CMV 1200 (lets say non precision approach).
RVR is 1200/175/175 or better, you can start the approach.
RVR not available, Vis is below 800m: cannot start the approach.
RVR not available vis is 800m, night, HIALS, then CMV=800 x 2 = 1600m: you can start the approach.
Hope that clears things up a bit.