You could try to use the one piece of liner, but if not, just go buy another piece. This way you can make the bog deeper. The bog plants should recieve plenty of morning sun on that corner. If you are growing veggies only 12' or so away, that spot is getting enough sun.
With the threat of no kids going swimming, I have a feeling that spot is going to be remodeled in a few years.
And back to the steps... They don't have to be that big, not a whole shelve running the lenght of the side or anything, (but that would be nice)... It just needs to be at the point on the side by the A/C. Right now it is a step yes, a 3 footer. And yes anybody could get out, but not without trashing the landscaping/rockscaping along the edge. Ladder time?