Flooring Tile - backerboard on concrete?

I may have really stepped into a mess...from one of my previous threads you might have seen that I'm trying to help a friend repairs some sloppy work by a contractor....things were going along smoothly until I started on the bathroom. The previous person stopped about 16 - 18 inches from the wall. Using 12" tiles this will require about another row of full size tiles plus a row of cut tiles..no problem. the problem is that the floor apears to be cement, but the area left exposed is not as even as it should be to lay tile on. It seems that the other person laid backerboard onto the floor (maybe because the whole area was not smooth?). I have a piece of backer board I could cut to fit this area, but how would I attached it to concrete? Is this normal? I have to put somehting down befor the tile because there is a height difference of the thickness of the backer board. Putting this down would seem to be an easy solution to the smoothness problem as well as the height problem...am I nuts?

USe thinset to attach the backerboard to the floor. If it's as uneven as you say, use a fairly large notched trowel and wiggle the piece in place like a piece of tile.