Torque to yield bolts are called that way for the yield point in the tensile strenght of steel. The yield point is the level of strenght a material can take before getting permanent deformation. Below that point, the piece will get back to is original shape when you unload it. For example, a spring should always be used below is yield strength otherwise it wont get back to is shape. A torque to yield bolt means that you should torque it just at the point that the bolt will extend slightly and will never get back to it's original lenght. I get this from my studies in mechanical engineering. While this doesn't mean that they WILL cause a gasket leak, I think that's enough to put them in the garbage and buy new ones. I wouldn't take take risk. It's clear that no manufacturer would recommend using them twice and a quality workshop shouln't either. Good luck dealing with these poor mechanic's !

(I'm sorry for my english mistakes, I usually speak french)