What meanings do the different parameters have to the SMAC processor? Which concrete models are used for processing?
All we can say to this topic is mentioned on the BEAM help pages at Data Processors -> Smac Processor
Do I have to perform the process individually for each band in the Level 3 binning processor?
No, since BEAM 3.4 it is possible to process mulitple bands.
In the processing parameters of the Level 3 binning processor, the Grid cell size is set to 9.28 km. Why?
The implementation of the processor in the binning mode is based on SeaWiFS algorithm and products. That's where the 9.28 km comes from. You can enter any other value here. For binning the bin cell size should be greater or equal the pixel size, so for MERIS FR you can use values >=0.3km. If the pixel size of your output product shall be less than 300m you should consider using the mosaic processor.