I'll answer these 3 questions as best I can here, but for more detailed explanations on how to convert your specific dataset, its probably best if you submit a support request to safe.com/support and click 'submit a case'.

In regards to Q1, yes you are generally correct. IFC is more detailed than CityGML, so there is necessarily loss of detail when you go from IFC to CityGML, including loss of some semantic details. That said, CityGML has some optional fields such as gml_name and gml_description which you can use to store IFC specific information that you may want to preserve.

For Q2, not sure I fully understand the question. Is your goal to preserve the volume information from ifcspace? You can certainly calculate the volume with a VolumeCalculator before you convert the geometry to a multisurface, and store this value in an optional comment field such as gml_description. You could also try preserving your room solids by writing out your room features as lod4Solid in addition to floor, all and roof surfaces.

Q3. 1 IFC floor slab typically becomes the ceiling of the room below and the floor of the room above. Since the slab is a solid, you typically need to break it apart into component surfaces and calculate the surface normal. The surface normals for the faces pointing up are for the floor surfaces, the surface normals pointing down are from the ceiling surfaces, and the surfaces pointing neither up nor down are typically from the wall surfaces. In the attached example, I used a PlanarityFilter to generate normals and then normalZ < -.2 to capture the ceilings (with the normals pointing down).

For some examples of IFC to CityGML conversion see:

https://community.safe.com/s/article/bim-to-gis-intermediate-ifc-lod-300-to-lod-4-cityg

and

https://community.safe.com/s/article/bim-to-gis-advanced-ifc-lod-200-to-lod-3-citygml

Example IFC to CityGML workflow from the Indoor Pilot