It's
very easy to see the witch. You should have no problem at all being able to see her. The single rider line goes to the right of the witch, so if you look to your left you'll have a perfect view of that whole regular queue room. The regular queue zig-zags in front of her and continues to the left. There were only a few people behind us so I stayed behind to watch her do her magic. My dad thought she was a real person at first!
You don't merge with the regular queue at all in singles - they fill up the rows with larger parties first, then adds the single riders. Imagine a rectangular room, the regular queue is on the left, splits into left and right (front rows and back rows, respectively). The single rider line is at the "bottom" of the rectangle. Unless there's more than one single rider for one row, you will most likely be on the left side of the truck.
I sat in row 9, 2nd from the right (normal queue), and in row 12, in about the middle of the truck twice (singles). I didn't have any problems seeing the final animatronic!
Hope that helps
