1)The bathroom is diagonal-across from the office (The bathroom is diagonally to the front of the officeもOK)
2)The fitting room is in the back of the store
って言えば通じます。
手前ー(店の手前) in the front of the store.とか!
gongkeさんへ the bathroom is across and down the hall to the left(or right).
the fitting room is at the back of the store
ってどうでしょうか??