I read that hardibacker needs an expansion joint in the middle of the room. They say that 1/8 space in hardibacker and tile above it? Is that right? What do I put in that joint? My room is 22 feet. What do I do around the edge or at doorway where tile will meet a hardwood floor.

Close enough. The spec is an expansion joint every 15 feet, and being that you'd only need one in that area, the best bet would be the middle of the floor. This is the first I've heard of the joint going into the underlayment, but it makes sense.