In general, the minimum connected dominating set (MCDS) of a graph is not unique.
Q1: Is there a conventional name for the set of vertices which are never part of any MCDS?
Q2: Has anyone ever encountered mention of this set in the literature? My search has turned up nothing, but I'm a non-mathematician who's new to graph theory.
Leaves are obviously in this (nameless?) set, but there are others, as well. Consider, for example, the paw or 3-pan graph, which consists of 4 vertices and 4 edges, and is obtained by joining a leaf to $C_3$. The MCDS is unique in this case, and it's a single vertex; thus, none of the other three vertices are ever part of a MCDS for this graph, yet only one of them is a leaf vertex.