In one of my previous operator (UK), for every flight the cabin crew were doing a counting of number of passenger per zone and the number one was giving me a paper with the 4 zone and each number of pax per zone.
it was really easy to check now with the load sheet the number of total passenger and each zone, easy to spot if passenger moved or was a mistake.
Most of the time some passenger had moved and sometimes when it was clearly impacting the CG to my experience we would ask the CC to move some passenger, for example, 3 pax from zone A to D ...
My last operator, we don't do that and it is even not mandatory to tell me the full number of pax on board (it depends of the destination, what I don't really understand)
If it would be mandatory for all EU (world) operator to do what I just said above with mandatory CC counting, this kind of incidents with CG would be unlikely to happen. But as always in the world, until we don't have a big crash at the end of the runway with media coverage and the world been shocked, nothing will change ...