Well the idea behind stop leak is to swell seals and "clog holes" that the fluid is leaking out of. There are many small holes inside the engine that cannot become clogged up. This is why I do not reccomend these products to anyone. What makes you say the engine is bad and not driveable?
When you say the engine went out, what happened? Will it restart?
Have you checked the engine oil dipstick to see if there is adequate oil in the engine? It sounds like the engine was starved for oil, possibly caused by the additive.
It may very well be. How does the additive starve the engine
I am not saying that it did, as I couldnt say that without looking at the car very thoroughly. There are small oil pressure ports that may have became clogged by the additive. Again check your oil level anyways you stated that there was an oil leak, any chance all of the oil leaked out?