AFrom 4.0 mm. up to 16.0 mm.  
AThe biggest difference between grades of chain is that carbon steel chain - Grade 30, 43, and 70 - is not recommended for overhead lifting, while alloy steel chain -Grade 80 and 100 - is recommended for use in sling assemblies and overhead lifting applications.
ADepend on the applications. 
- Galvanizing is suitable for highly corrosion resistant applications.
- Zinc Cr3+ plating is suitable for conveyor chains in the food industry.
- Zinc Cr6+ plating is for ASTM, JIS international standard chains. In some countries, they define colors of chains for different applications.
AThere are standards for how thick coating should be applied on chains and manufacturers don’t publish this information. 
ACarbon steel or alloy steel wire rods, depend on grades of the chains -grade 30, 43, 70, 80 or  100.
AYes, we issue quality certificate for international standard chain and product certificates for commercial chains - heavy duty chains.
AYes, only for the applications with proper size and grade. Anyway, the chains still need to be replaced periodically.