Instead of scattering her litter around the yard you could try just putting a box on your porch/steps. Once litter is spread it losing it's "potency". It may be more effective to be left in one area rather than spread.
When you went to the Oakville HS did you ask to see the feral and sick rooms as well? It could be your kitty acted feral because of fright so she would have been put in that room. Unfortunately they are also the first to be euthanized. If she acted feral they probably did not check for a microchip. Cats can be very friendly when in a familiar environment but turn absolutely wild when in unfamiliar territory.
Is there any new construction going on around your area where your kitty may have wandered into an area and been closed in? If that is the case it would not hurt to check it out too.
Good luck. Checkers looks a lot like one of mine in the first picture.