I still think it's a tricky "test" to use a plastic tupperware container to test for soap... it's possible it had soap residue from the last time it was washed? Or even if it's bought new from the store. Is there any way you can run a similar test using glass containers?
I got your PM, and I still believe this is a current topic and one worth continuing for the benefit of everyone.
I'm also impressed with Petsmart's policies about returns.
Can you confirm that your filter has activated carbon in it? That usually catches contaminates which are flowing through the water. (But won't remove something that's adhered to a surface.)
I actually think in a lot of tanks there's something that ends up in there from something, probably our own hands. So there could always be a minor amount of soap or something else that gets in the tank, but hopefully the filters will assist with that. At this point, I would proceed forward and wait for something disastrously evident before you pull the tank down again.
I'd continue with the cycling, unless you see foam in the tank itself, etc. When the cycle is complete, I would do many water changes. As recommended above, work on the dillution part of it. Maybe even add aquarium salt to it, because I don't think it will hurt the bacteria colonies, and gravel vac with the salt, get anything off that might be stuck to something so the carbon can grab it. Do lots of major water changes, flush it all out, (which you'll have to do to reduce a large number of NitrAtes anyway.) After that, I'm betting you'll be in fine shape. It might be a good idea to start out with just a few fish, and see how they do. I'm thinking it will be fine, if your plants are ok, and you haven't seen signs of contamination in the tank. To be honest, you used such a small amount anyway, with the amount of boiling and flushing and rinsing and replacing of parts, you are probably on your way.
One question I've asked though that you haven't answered yet, was whether you tried anything other than the hot water? Did you use the suggested baking soda, salt, or bleach? (which was recommended to me, but I was too timid to try. I've heard it just breaks down after a period of time.)
I am rinsing the gravel yet again tonight, and yes after I boiled it I did rinse it down with a hose two times, but am doing it again tonight. I am going to be testing small amounts of gravel at a time. One way I've been testing the water/gravel is by putting it in a plastic container and fill the one with gravel in it with water. I then shake it a few times. If there is no foam or bubbles that take long to go away, then I know it's all good.
