Gus, I can't imagine where that "hot dog stand" story originated. There's no shortage of photos showing Paul Castellano and Tommy Bilotti dead in the street right in front of Sparks, with the Sparks sign clearly visible.
Gotti and Gravano were waiting in a car nearby when the murders were committed, but they didn't pull any triggers. Gotti originated the idea of whacking the duo, and Gravano supported him. Actual planning was done by Frankie DiCicco, who had been a capo under Castellano but agreed to sign on with Gotti and became his consigliere after the hits. He was later blown up by a car bomb intended for Gotti.
BTW: most restaurant critics agree with your view of the food.