Use light cache for glossy rays is great way to reduce render time, but sometimes we have artefacts with it.

There is simple way to avoid it, it is to turn Off prefilter and On Filter Fixed in Light Cache Reconstruction_parameters with value if Filter Size 2-3 times bigger then Samples Size of Light cache.

Turn on in your GI reflective caustics often adds only problems. But there is a bypass maneuver, which offered a caustic little bit with a small (about 30%) increase render time but without any stains! To do this, check DE (Detail Enhancement), and to calculate the LC and Ir map in to file with switched off reflective caustics, and before render turn on casuistic and use from file LC and Ir map. Settings of DE will be responsible for the quality of caustics. For a large resolution could be left default value. For the smaller resolution it`s possible to use world and set the desired radius mm. 30-40 mm. quite enough, and fairly quickly.

Lights spots on the ceiling - the best way to get rid of them, it place the land behind the window. I mean to put outside the plane near the window with a dark (the more darker the less light on ceiling) material. You can give a little color and color of light on celing will be the same.