Thanks for your patience.
I agree that the need for the reference to 3.1.1 is not particularly clear here, since it only defines what an area is and does not address use or non-use of texture or a key. This reference may be removed in the updated version of the Supplement.
I don't believe there is a specific guideline about when to key or not to key a texture. My understanding is that if the texture is not used symbolically, to represent something, but serves simply to distinguish one area tactually from surrounding areas, then a key is not required. Indeed, it would be difficult to state on a key or symbols list what the texture is representing.
If a texture is used to stand in for something (i.e., water versus land, categories in a bar graph, varying amounts of rainfall) and a label or alphabetic/numeric key would not fit within the textured area, then an explanation would be required either in a key before the graphic, or on the Graphics Symbols Page following the Special Symbols Page (see 5.14 for the section on Graphics Symbols Pages).
I hope that helps!