"The question remains what the "Tropidonotus tessellatus var. flavencens" Werner 1891 really is... just
a widespread colour morph, a very local colour morph, or what?"
If the morph was found in Italy, it should not be a very local colour morph (I guess that Mebert is referring to specimens which look similar as the one found near Zadar, so typical var. flavescens=no "borderline" case. Only an assumption), and therefore, not restricted to this Croatian region.