Yes. With Real Lights on, the addon adds Blender light objects at the lamps and daylight through the windows, so the interior is lit in both EEVEE and Cycles. Lights On, Light Strength and Daylight are in Materials & Lighting.

The PBR sets are CC0 scans from Poly Haven, and the book spines, signs, labels, pictures, globe and rugs are generated for this addon and also free to use. You can use the results in personal and commercial work. The addon code is GPL.

Yes. Each design is one mesh with UVs and PBR image textures (base colour, roughness and normal maps). Use Convert to Mesh, then export as FBX or glTF. The wear and dust overlay is a Blender shader effect: bake it if you want it in the engine. For real-time use pick Detail Low, lower the book fill, or export single pieces such as a bookcase or a reading table.

It depends on the size and the books. At Medium detail a 150 m2 room is about 450,000 to 850,000 faces depending on the shelf layout, the 308 m2 Reading Room with about 16,000 books is close to 1 million, and the largest halls with over 40,000 books pass 2 million; single pieces are roughly 4,000 to 60,000. A rebuild takes a few seconds for the big rooms. The sidebar shows the face count, and Detail Low, a lower book Fill or switching the books off cut it sharply.

Yes. Edit Floor Plan opens a flat plan in Edit Mode: the face is the room outline and every loose edge is a bookshelf. Move, extrude or delete them with Blender's normal mesh tools, press Apply Plan, and the library is rebuilt from your drawing. Back to Automatic returns to the preset shapes and layouts.