Seems like it’s a special case scenario,you should check those meshes and make sure you don’t have any normals issues or other mesh errors. Ligthtmass bounces shouldn’t make a drastic difference like that. Just make sure environment color in world settings is set to black.
You don’t need light blocking volumes since everything should work as is and lighting should look more or less correct. If you need to block a certain area with specific shadows you can use any object just set it not to dispaly in game but to only cast shadows. You can even use maps on the lights themselves.
The exposure settings is in post process volume under eye adaption or auto exposure. check docs.