Are all shower arms removable

I have one very similar to that. It lifts out vertically.

There are six keyed slots in the drain tube that the cover slots into. So you have to lift it keeping it level. Place something under two sides of the cover and lift together, if you only lift from one side it will twist and won't come out.

There will then be a plastic hair trap guard that just lifts out easily that you can clean.

When a 2-year-old shower is leaking, something is definitely wrong. Call the builder, and tell him to fix it.

Yes, there's supposed to be waterproofing behind the tiles. The tiles and grout by themselves won't be enough to keep the water out of the wall. I don't know if there are precise rules for how high the waterproofing should go, but I would think at least as high as the shower head. When I built my shower, I went all the way up to the ceiling.