Is it difficult to change out the wheels when needed?
Not too complecated. You only needs are 9/16 and 7/32 allen wrench
What is the metal on the wheels made of?
zinc plated steel.
Does 'aluminum diamond plate' mean the plate is aluminum through & through or metal that is plated with aluminum?
Yes,the plate is made from aluminum plate. Casters are zinc plated wiht rubber wheels
What is the thickness of the aluminum diamond plate?
Plate thickness is .190" made from 5052 aluminum diamond plate
The wheels do not appear to be black, reason I ask we have had problems in past with black rubber staining our deck in the hot Florida sun.
Correct. The casters on the square plant caddies are a light gray