Indeed 480855836 is correct as "base reference" follows from this all the other levels are correct. 

So for my first concern about POI correct level is -2 even elevator indicate something else. 

I just looked whet "Carto’Cité" used for their mapping of SNCF stations and they add https://wiki.openstreetmap.org/wiki/Key:level:ref 
(they used for indoor=level) but i think that can resolve a lot of that case in Brussels Metro 

Simple Indoor Tagging wiki page say : "the number or code of the level. This models the level numbering scheme as it is displayed in the building itself, e.g. on floor plans or elevator buttons, which may be different from the level=* value. " 

Do you think it's a good solution ? 
