Can the 16 x 22 niche be installed sideways without the shelf and still have the slope be correct?

These are designed to fit between standard 16 .in wall stud spacing. If you have a larger wall stud spacing and want to use sideways it will work. There are no slopes built in on the niches. Once the niche is in place you just back butter your sill tiles back edge a little more generous to create a small slope for water to shed more easily.