OK, I'm going to give this a shot. Are these the same booties I helped with before? I've been hoping to get to see how they turn out!
OK, I think you're right about the 18 stitches adding height to the back of the shoe. After those 18 stitches, you're going to cast on 22 more stitches, right on that needle, that will be part of the same row as the 18 stitches. That's the strap.
Now, I think, you're going to turn and knit back across those 22 stitches and the 18 stitches, and then cast on 4 on the other side of the 18 heel stitches. I'd have to see a picture to be sure, but from the info you gave, that's my interpretation. I think that's where you will sew the button. Then you knit back across the 4, the 18 and the 22 up to the last 3 stitches, then do the button hole directions. Then you'll knit 2 more rows, then cast off.