Yes, 0.2 is the maximum. However, in most cases we don’t advise to use a value higher than 0.08, because it will make the scene lose dark shadows.
1200 is a very high number of samples. It should be enough for your scene.
The artifacts you’re seeing are caused by limited lightmap resolution. The default lightmap resolution = 75 means that light is samples 75 times per meter, so one lightmap pixel overs roughly a 1.3cm x 1.3cm square. Set Custom lightmap resolution in the Objects tab to a higher value for this object to improve the lighting precision for this object.
The Max lightmaps setting in the Bake tab limits the number of lightmaps in your scene to 2. This is too few for your scene to obtain the desired Lightmap resolution set to 130 in the Bake tab, so Shapespark automatically reduced the lightmap resolution for all the objects in the scene. You can check the reduced lightmap resolution in the Objects tab as on the below screenshot:
Try reducing the global lightmap resolution (Bake tab) to 75, it will leave more space in the lightmap for the objects you want the custom resolution of 130 for.