Assuming that a and b are correct, is the construction of c correct? Also are there better/simpler ways of conveying the same information?
a) las cortinas en toda la casa son grises. (in every room)
Meaning: "The curtains throughout the house are gray".
b) las cortinas en todas las casas son grises (in at least one room of every house)
Meaning: "The curtains in every house are gray".
c) las cortinas en toda la casa en todas las casas son grises. (in every room of every house)
Intending to mean: "The curtains throughout every house are gray".