You (TD) said [in answer 2/3]:
"When critics try to interpret Conceptual Art with the help of concepts of the esthetic sign then they install Art & Language on the periphery of their discussions."
But what about an interpretation of Conceptual Art that approaches the works broadly semiotically, rather than aesthetically or poetically? To the extent that the works utilize objects and materials, these function as signifiers of meanings outside artistic intention or stated function that shouldn't be ignored, particularly if the work is held out as ART, which is a place where utilitarian/functional concerns are often suspended. The alternative (to ignore the physical aspects of the work and focus only on the discursive or conceptual content) would seem to imply a form of Platonism that privileges the concept while demoting the object to the condition of being a mere shadow of the idea--a passive vehicle for conceptual content. The works of Art & Language are as amenable to this (semiotic) sort of analysis as one could hope for, and could conceivably occupy the centre of one's discussion of Conceptual Art, rather than the periphery. All of this is not to imply that the intention behind the work is unimportant or of a lower order, but rather that there can be no immunity from the semantic density of the signifier, such that one should only consider the signified.