1. How do I assign an old lightmap back to object? I read one should just drag it to inspector section but there's nothing about lightmaps.

2. How do I bake lights for single selected object without loosing previously baked and assigned lightmaps?

3. Sometimes my calculated lightmaps cover just 1/4th (bottom left part) of the texture which is great waste of space. Is this just about bad UVs or what?

4. Have I understood correctly that in oder to get multiple objects' lights together in one map, I need to combine all the objects just to one mesh with one UV map?

5. Is there any clever reason why Beast unassigns previously set baked and set lightmaps for all objects if I try to bake just one cube?

I have googled one day for tutorials and material but couldn't find anything clearly telling about these.