Thanks a lot for the advice, adding “landuse=residential …” rule helped me with the problem (B), but I still got problem with the sea generation.

I tried the both options you suggested and I still have the tiles that contain no land green… I think it’s understandable, since if these tiles contains no coastline, mkgmap can’t know if it’s whole land or sea - I still don’t know what to do with it? Isn’t it necessary to somehow combine the .osm files rather than converting each of them individually into *.img (which means also that sea is generated for each file individually)?
Also should my mkgmap custom file contain something special to support the sea generation?