There is no requirement to be visual with the runway at any stage of a CATIIIB approach and landing with no DH. If you are using a "mini DH" which is normally 25ft (Leeds Bradford, anywhere in France) you need to be able to see one centre line light.
Once you're passed the OM/1000' point, the decision is made.
Spot on, assuming no relevant failures you are going to land off a CAT3b with no DH. Once you have passed that point with a legal RVR you have satisfied the regulations and you can continue the approach to a landing regardless of the what the RVR drops to.
Quite logical if you think about it. RVR is not uniform across even quite small areas and can fluctuate rapidly. If you are given 75m at all 3 points it is quite possible that the RVR at the runway turn-off could be 50m or 90m. The threshold RVR can often be very different from the reported viz. which is why lots of CAT1 approaches to 550m/200ft end in go-arounds, you can't see the runway at DH despite the fact that the reported viz is at or above minimums.
The regulations recognise their own limitations and that of the measuring system which is why they allow you to continue to DH if you have one, or to land if you don't, once you are past the OM.